I understand this topic has problem been discussed a million times on here.
Attached are some pictures of the wheel arches on myx5 nc.
How hard or easy would it be for a complete novice to repair them?
Really depends on what you find underneath, bubbles are usually the tips of proverbial icebergs, if she were mine I would get stuck in there, but only because I know the cost of not doing so for any prolonged length of time can be financially crippling. Good luck 
That won’t just be on the surface. It will need the best part of a new wing.
In that first pic you can probably get away with doing a DIY job, it’s easier and won’t get noticed so much. I did mine around the arch lip to treat a couple of thumb size bubbles in the paint on my previous car (see pic)
The other ones are tricky to get matched in with rattle cans, especially silver or a metallic type finish. Depends how picky you are. That wheel arch lip looks like it’s been painted before to me.
I’ve just fetched mine back this very week from the body shop, no rust just some scrape and stone chipping to deal with.
When I had an estimate the guy said he was apprehensive about MX-5’s as the last one he did proved to be more than just a blow over with paint.
Until you dig into it you don’t know what lurks beneath.
Thanks for all the replies, can anyone point me towards a video that can show me how to do it?
Take a look on here . Mk3 MX5 Rusty rear arch repair - NC - YouTube