Do what I do…. Take the negative lad off each evening. It takes just 2 minutes ….your problem will not get solved by doing this. But you will avoid the (unexplained) drainage until you sort the problem out…
If you do this. You will find that a couple of lights appear on your dash board But. They will go out once you “ next “ start the car… Each time you take the lead off The same lights will temporarily appear on the dashboard …… For some reason MX5s do tend to have battery problem
So would I be disappointed. But you have a lovely car there. It will get sorted It is annoying but I’d hate to hear your ‘ choice words ‘ if you owned a Mercedes SL or CL circa 2000/ 2005. M friend is still paying out to get his SL sorted out. £7000 to date ! And still got a £2000 bill ahead… best regards Ray Epping Essex
Anybody owning a mk4 (ND) should invest in a smart charger, either one of the expensive ones or one from Aldi/Lidl. Mine lives on a Lidl one never had a problem with flat battery, only thing is that if the power goes off you have to remember to reset the charger.
Should add that if we have been away for the night in a hotel etc the car is no problem to start next day.
Ah.
Ignore me.
Missed " Brand new Battery".
Apologies.
I’m running in my Nan’s Boots Reedies for her. 
My first MX5 was a 2006 NC 1.8, always had this problem even after a new battery, had it in the auto electricians for 3 days and he couldn’t sort it. I ended up buying a trickle charger which I always left connected when the car wasn’t in use.
I now have a 2012 NC sport tech 2.0 which has exactly the same problem, gave up worrying about it and have it connected to the trickle charger as per the previous car, I have seen so many alleged fixes for this problem but no longer care about it. The car is our second vehicle and so hardly used, it has never let us down whilst out on the road, only if left for more than 3 days at home without the charger connected.
Use a multimeter on the 20v range, lower if it does not show a voltage and measure the voltage across all the permanently live fuses - there are charts on the internet to show what voltage across a specific fuse rating equates to current, but pretty much any voltage will show which circuit is drawing current.
Voltages should be in milivolts if at all and will normally only be on the clock, radio and alarm circuits - pull any fuses that show voltage and see if the problem goes.
Thanks Derek 
And Welcome….
Update #2…
My 5 has been away for a week and I think there has been quite a bit of head scratching from what I can gather!!
After finding a drain on the fuel pump, they found that they needed an ‘injector rail’…this didn’t solve it, and the pump still injects fuel when not needed.
They are now saying the ECU is at fault, and going to replace. I’m aware that the mileage will change and have to be documented, but are there any other problems that anyone could highlight? Just in case?
Ian
Is this a Mazda dealer carrying out this diagnosis? Nothing of what your saying they are saying makes any sense to me.
No it’s a garage close to where I brought the car from. I’ve been assured that the chap fixing it has a lot of experience with 5’s…but I suppose that’s open season on opinions from members here? I’m a ‘noon’, but I also don’t want to be fobbed off!!
*noob
Ultimately they are looking for the cause of a battery drain yes? Have they confirmed that there is a drain on the battery and are they suspecting that the fuel pump is running while the car is switched off which is causing the drain?
Hi guys. NC Sport 2.0l here. Have had similar problems with charging. My car is used infrequently & it’s not always possible to give her a good run to keep the battery charged. After a lot of head scratching & using the wife’s car to jump start her I opted for an RAC solar trickle charger plugged into the obd 2 socket. Been used since January & even with the short daylight hours I have had no problems. I did, however, give the battery a decent time on a smart charger in case of sulphate build up before using the trickle charger.
Hi Robbie
I’m understanding that replacing/fixing the fuel pump was what they thought would fix the problem.
It seems that having done this, there’s still a drain and they’ve narrowed it down to the ECU?
At the moment I’m only going on what they’re saying.
Just pop the Negative lead off the battery ! ( A one minute job ) It’s a strange thing that Batteries drain so quickly on some cars. Mercedes take some beating in this regard … These drainages are hard to find on modern cars But seem to be avoided by just taking one battery lead off ( negative )…. When you re connect the battery you may ( or may not ) get a warning light come onto the console, That’s just the computer sorting itself out as the power comes back on. Once you turn the ignition off and on again the light will go out, This should stop your battery draining ( usually through lack of usage ). Best regards from Ray Hill Epping Essex 
Problem with removing lead is some 5’s including mine take an age to relearn its values and you may have lumpy idling so not very practical or enjoyable.Thats why i stopped using my isolator switch.
Final Update!!
Sorted 
Finally got her back in the week and now has 10k less on the clock and she seems to run smoother?
Wether that is to do with the calliper or the ECU, I’m not sure?
I’ve read that the ECU takes a while to ‘learn’ to idle smoother?
My only real concern is that whilst braking gently, she seems to ‘lurch’ a bit? Wondering if the fact that the calliper was seized for so long on the disc that it has damaged the disc?
Onwards and upwards with the summer months imminent
Yes, you made a valid point there, But the Computer does quickly find its original settings So it’s a case of Pros and cons “ is the Battery problem more important to resolve than the blip caused to the Computer settings, It gets resolved in just a few seconds with my car… on an off topic note, There’s a Retired meeting to be held at the old and quaint Essex pub The Woodman Just outside Ongar Essex. On the 15th June at 12 o’clock ( sharp As the pub gets very busy and we need to get parked and seated before the throng of others arrive, guess what. I have commanded the “ overspill car park for our venue. We can look over the farmers fields whilst admiring our little treasures in the nearby overspill area, Good eh ? Please try to make it, Best regards from Ray Hill Epping Essex
Yes Bruce… point taken. But with my particular car The idling settles down after a quick run up the lane To me the batter drainage is primary to my concerns. ( I hardly ever use my car ) sad eh ! So Battery problem becomes top concern … Take care Bruce keep safe hope to meet you at a club meet one day …. Best regards Ray …. Epping Essex
Mine take arounf 15 minutes left at idle before settling down and never did by running it for a week. After reading lots of threads on the subject seems some people’s settle straight away some don’t.