Mountain bikes... again!

Right I posted a while ago that I wanted a mountain bike but needed info on what to get, and then got leant one indefinately by my chick's bro.
Well thats all cool but it still hasn't quite hit the mark, the bike is an old(2000 maybe?) Claude Butler Cape Wrath which is cool but its way too big as its 20 or 21" frame, and I'm only 5'8". I can ride it ok but the seat is right down as far as it can go, and its not mine so I don't want to break it so I can't go silly on it.
I still have gear lust tho.
I have read up on mtbs and now know that I need a GT Avalanche Expert 2006 for £550 reduced from £800.
At first I felt this was way too much to blow on a bike but its all ok now as I have stared long enough at pictures of 2, 3, and even 4 grand £ bikes and so £550 is nothing really
!
...but how do I convince my chick that this is what should be without incurring some sort of cruel or painfull retribution? Erm...also bearing in mind we just bought a house and it does need a wee bit of work doing to it. Like the bathroom. Then the kitchen. 'N' stuff like that really
.
Yeah so come on guys, how do I help her out and lead her into enlightenment?
Bike here :- http://www.paulscycles.co.uk/products.php?plid=m1b0s2p98&z=184
Well thats all cool but it still hasn't quite hit the mark, the bike is an old(2000 maybe?) Claude Butler Cape Wrath which is cool but its way too big as its 20 or 21" frame, and I'm only 5'8". I can ride it ok but the seat is right down as far as it can go, and its not mine so I don't want to break it so I can't go silly on it.
I still have gear lust tho.
I have read up on mtbs and now know that I need a GT Avalanche Expert 2006 for £550 reduced from £800.
At first I felt this was way too much to blow on a bike but its all ok now as I have stared long enough at pictures of 2, 3, and even 4 grand £ bikes and so £550 is nothing really

...but how do I convince my chick that this is what should be without incurring some sort of cruel or painfull retribution? Erm...also bearing in mind we just bought a house and it does need a wee bit of work doing to it. Like the bathroom. Then the kitchen. 'N' stuff like that really

Yeah so come on guys, how do I help her out and lead her into enlightenment?

Bike here :- http://www.paulscycles.co.uk/products.php?plid=m1b0s2p98&z=184