Mountain Bike - second hand buying advice £3-400

Hi all,

About 2 years ago I semi-got into road cycling and love it. Entered a few rides, L2B, London100 etc and love those also. However, winter is definitely not the same as summer haha. With that in mind, me and a friend are looking to blast about in the mud over the weekends on a mountain bike - trails mainly, nothing too hilly, but all off-road stuff, puddles with a few small downhill runs (we live in Sussex...)

I've been trying to do a little research, but to be honest, it's quite daunting. I recently sold my very old GT Aggressor XC1.0 (£125) and am looking to put the funds towards a replacement second hand bike.

I think I only want a Hardtail - I don't feel I need full suspension, and putting the money into a bike that is trying to cover all bases doesn't seem to make sense?

I have a budget of around £300 give or take, and really don't mind how old the bike is, but I'm obviously looking to utilise the second hand market for making the ££ stretch as far as possible. I'm not interested in new - I just don't think I'll get anything really worth while for that investment.

Couple of options so far:

2014 Boardman Comp 650b - £250-£300
GT Avalanche 1.0 All Terra - around £400 (not sure on year)
Giant XTC SE - £200 (not sure on year)
Diamond Back Heist 1.0 - £350

Happy to listen to any advice. I'm sort of mechanically minded (to a point), and don't mind A: learning and B: getting my hands dirty