Just thought I’d do a little folow up now that my new battery has been fitted…
I ordered mine from Eurocarparts with next day delivery and it came very well packed.
It’s a Bosch S5 Car Battery 063 5 Year Guarantee for £80.94 including next day delivery.
It has vent holes either end with a blanking plug built into the terminbal caps so ypu can plug one of the vents (not needed in the MX-5 as they have vent pipes for both ends).
Once you’ve pulled up the carpet and piece of board with the jackl handle attached it’s pretty straightforward to remove.
First I removed the negative terminal clamp with a 10mm ring spanner (forgot to take photo) then a wrench to undo the retaining bracket bolt (14mm IIRC)
The clamp can then be unhooked from the hole in the frame.
The 2 vent pipes either end just pull off.
Then undo the positive terminal with teh 10mm spanner (again I forgot to take photo).
As you can see the physical dimensions are quite different.
The original plastic tray for the Panasonic tray needs to be removed as well as it wont fit the new battery. This isnt attacjed in any way - it just needs lifting out.
I’d bought a battery absorbing mat since I no longer had the tray so placed that in the hole.
Its a bit of a fiddle to get the bigger battery in place taking care not to catch the terminals in anyway or trap the vent pipes. Once in I attached the positive terminal first, followed by the negative terminal.
Frustratingly the clamp isnt the best fit for the bigger battery. I had to move it up a hole in the series of holes in the frame and due to the postioning of the carry handle you cant quite get the battery to clamp like the original. I tried a few different combinations and pic is the best of the options. It’s certainly not moving about and I will check it in a week or 2 to make sure.
With regard to the vent elbows when I asked at a local parts factor they just gave me 2 spare they add but I did see you can buy them on eBay etc.
After a few trips I attcahed my battery/alternator tester on it and all is looking good 
BTW As a few posters have mentioned above if anyone wants a 4 year old panasonic battery and collect from North West (I will NOT post so don’t ask) DM me 