I was in your same position about a year ago when I first got into mountain biking and was unsure as to what bike to buy. There's so many to choose from out there that knowing which bike to get can get abit confusing.
The Claud Butler bike you mentioned sounds ok for the price your willing to pay. Basically its a very, very entry level mountain bike that will get you round the easier trails and see you right on the roads. If like you say, your only planning on doing the odd dirt track twice a week, then yes this bike will be fine for you.
However, as soon as your start riding a couple of times, there's a good chance you'll get into it more and therefore want to go on more demanding, longer trails (theres plenty to choose from around Sheffield), and therefore theres a good chance you'll wish you had spent more money on a better bike. Paying around £150/£200 more will get you a hell of alot better bike (Disk brakes, better forks, lighter frame, etc).
See if your work has a cyle to work sceme. If so then your laughing as you'll be able to get a decent bike for next to nothing.
Hope this helps?
Oh and if your wondering, the bike I ended up getting was a Rockrider 8.1 from Decathlon. £500 - beauty!