DIRECT LINK: http://www.decathlon.co.uk/EN/rockrider-5-0-men-s-2010-138413764/
Every cheap bike deal posted on HUKD creates a lot of argument, often because full suspension bikes are offered at impossibly low prices (ie impossible to sell a decent, well made bike with all the gizmos for such a low price). These bikes catch out the unwary but are almost guaranteed to disappoint: horrible to use and inherently poor quality (some would claim dangerous too).
I post this as an example of what can be bought for (just) under £100.
It has a fairly basic spec but comes from a well regarded retailer and is supplied built up and tested. Components are from respected manufacturers, eg Shimano 21 speed gears, KMC chain. The frame has a 5 year guarantee and the other non wearing parts 2 years.
Three frame sizes are available and there is a sizing guide with the details on the link.
Winner 'Best Budget Bike 2010' on the Gadget Show (does that mean much?).
Decathlon are Europe's largest sports retailer and this is part of their own brand range: no doubt they achieve this low pricing through enormous buying power.
If there is a downside to this deal it is that Decathlon has a fairly limited number of stores in the UK (8 I believe, incl N Ireland), though more are opening and they will deliver (see site for details).
If you are looking for a bargain bike I really think this is likely to be the best start point for something much less likely to disappoint.
PS It's well worth browsing the enormous range of B'Twin bikes on the site too.