How did you get on Matt?
I use my standard road going 1.8 MX5 for hillclimb, really good fun and yes, I am occationally up against stuff I can't beat (this weekend a A3 Quattro Turbo running around 250 bhp) but depends on the hill, I was only a few seconds slower but had a great time. Only 1.2 seconds slower than a Clio 180 Sport on R888's but almost 2 seconds quicker than another standard 1.8 MX5 and 3 seconds quicker than a BMW Mini Cooper Supercharged. Hillclimbing for me is as much as beating your competitors as it is a personal challenge. I set a time I want to beat after my first practice run (if it is a new hill) and push as hard as I can to achieve it. I run Falken ZE912 tires which are excellent but the 2 guys who beat me were running R888 Toyo road legal track tires (one drives to the event on them).
So join a motorsport club to understand what is required (timing strut, yellow tape on -ve battery wire, arrow to show direction of ignition off, approved helmet/overalls/gloves, numbers) Woolbridge (£15 per annum) has info on starting hillclimb worth a look. You can also call up the club scrutineers for helpful advice - they really are there to help not just fail you for a laugh. The competitors are the most helpful I've ever met so help is always on hand.
Also have a look at the HSA as they have an MX5 championship.
Hills vary a great deal, again have a look at Woolbridge, their main hill is Wiscombe park, which is a fantastic hill, very technical and fast. Just back from Manor Farm Hillclimb in Charmouth, couple of videos attached.
'1994 MX5, UK car, T R Lane GP Roll Bar, white Mk2 wheels, K&N Panel filter, KYB Adjustable Dampers and lowered springs....and a Mazdaspeed badge!