Hi Morgs,
I’m defo no bike expert, still learning after a few years.
With a budget of £300 you are not going to get anything amazing, certainly no XTR kit. You could risk ebay and look out for a little used hardtail.
Few years ago I was in almost the same position as you, didn’t want to spend much but wanted to get into biking again. I bought an Evans brand bike, Pinnacle Evolution. Although it was over your budget at 600, I sold it less than a year later for £350. My mate also sold a year old rockhopper for about £350….so if you know what to look for ebay might be better than a new one.
If you want to go new I would say look out for the best components, not sure what disk brakes are like at that level, they might not even be hydraulic so you could be better going for good V brakes.
Just to give you a couple of ideas…..
http://www.evanscycles.com/products/trek/3500-2010-mountain-bike-ec020717
http://www.evanscycles.com/products/kona/lanai-deluxe-2011-mountain-bike-ec026529
Just do loads of research and find out all you can about what you are buying.
Hope that helps and enjoy getting back into it!