So, the oil was last changed in my Mk1 in late 2016, this is something I plan on changing since it will have degraded over time.
The oil filter was likewise changed then, but the car has only done 35 miles since it was fitted.
Should I also change it as a matter of course, or will it be perfectly fine given the low mileage inspite of the years?
Personally, after 7 years I would change it. It will be full of 7-year old oil, and it may have degraded.
It’s a relatively small cost for the peace of mind.
I assume that you’ll be replacing all the other fluids (especially brake fluid)?
I figured that might be the case and I admit I was leaning more toward the peace of mind aspect but figured I’d ask.
That’s the plan 
I agree with the replies thus far.
Definately replace the filter if it has had 7 year old oil in it, irrespective of mileage.
Furthermore, I always fit a genuine MAZDA oil filter, as they have the right non-return valves in them to help keep oil up in the top end of the engine, to aid the camshafts on initial start-up.
After that long out of use I’d want to pull the plugs, put a teaspoon of oil down each cylinder and give it a turn by hand before running it. Along with fuel and other fluids of course 
And the tyres!
I’ve checked back through my documents and it’s my memory at fault as it was 2019, not 2016 it was last serviced. It didn’t help that I found an invoice for tyres which was from 2016, but turned out to be for the spare.
I never knew that, I know it currently has a cheap aftermarket one fitted but I was going to go genuine Mazda if I did buy a new one as I’d seen some complaints of leaks with some aftermarket ones. Still interesting to hear about the valve though.
I admit I never even thought of doing that the few times I’ve started it up - on the plus side it always sounded normal.
I know tyres can rot from inside out, but at least externally they appear fine, still hold air etc. and I can also confirm now that four new tyres were fitted in 2019 along with a battery (which I’ve taken out a few times to keep topped up and is still good).
The cost of an oil filter means that this is a rather pointless question. Just buy a Mazda one and start looking at more important items that may have deteriorated over time.
Even the cost of an oil filter, if not required, would have meant more money spare for the other items that need replacing which is why I decided to ask as, even though I was fairly sure what the consensus would be, every penny saved etc. etc.
An oil filter is £2.24!!!
…yes…
Not from mx5parts - £6.84 (Mazda filter, and that’s with the current 10% sale).
Though if your price was also a genuine one as well, where did you find it at that price?
Oil Filter, Aftermarket, MX5 Mk1/2/2.5 – MX5 Parts
£2.24—£6.84 is irrelevant, the cost is completely insignificant when looking at the cost of running the car and should not even be under question.
I bet you can open your wallet in your trouser pocket with a boxing glove on. 
Penny pinching over an oil filter seems rather, well daft and false economy to me. 
Just an opinion, do a full thorough main service and all will be fine.
yet more expense 
(would need to buy one ).
Would you like me to blow in your tyres to save you the 20p garages now charge to pump your tyres up.
35 miles in 7 years (five miles per annum) Have you been away somewhere doing a stretch?