As above, at £300 avoid full suspension at all costs. The bike will weigh a ton, the linkage will use crap bearings and the rest of the component set will be compromised to offset the cost of the frame.
With respect, having been out of the saddle for 15yrs, you aren't going to be doing anything that needs full-sus. If you want comfort, get a decent saddle, not a cheap full-suss.
I'd also suggest going second hand. If it must be new, look for '09/'10 models that have been reduced.
IMHO, at this price point, you wont go wrong with a Trek or a GT - Specialized and Kona are good too, but choice will be limited at this end of the market.