Just found this…
http://www.paulscycles.co.uk/products.php?plid=m1b4s24p2388&tbv=GIANT_ARETE_1_WOMENS_Mountain_Bikes_Womens
OK, it blows the budget by £150, but it’s a full bike, with Reba’s, SLX/XT, lightweight own brand finishing kit (Giant’s kit is always amazing for the money) and all in a womens specific geometry frame (it’s basically the XTC above but with specific geometry). Worth understanding how tall your Mrs. is as ladies bikes are usually quite short for their size, unless she’s sub 5ft, she’ll probably need the 16″ version. Over about 5ft6 and probably onto the 18!
Geometry here
Just a note, but CRC have a few of the frame only for £250 here… The RRP was £400 frame only, so it’s a pretty good lightweight XC frame anyway.
Other than that, the Rock Lobster’s C-G says above are good, probably the Tig Team SL as it’s not much over 3lb and comes in a fairly small 15″ size… Or there’s the On One Scandal for a bit cheaper, comes in a 14″ size. There’s also the Kinesis Maxlight KM-210L, another female specific frame that is by all accounts very light, comes in 4 sizes too.