I have a couple of 2litre Platypus' but only a 35litre rucksack. Rather than going to the expense of buying a Camelbak is there a suitable small Platypus compatible ruckie available?

Salomon Raid 200 is a 20L hydration system compatible rucsac. Nice and light, plenty of pockets, well comfortable.

try the gelert range, normally available at camping shops etc, they retail at about £17.00 and have a 2lt bladder and pockets for tools phones etc,

The Vango Fury range is very good as well, with sizes from 14 - 35 litre & can take upto a 2 ltr bladder (I've managed to squeeze a 2.5ltr into my Fury 14 & it still has space to spare for all the other stuff)

Cost £30 and comes with a very nice set up to stop your back getting hot & sweaty.

Berghaus Bladdered is pretty good, same(ish) size as the Mule:

http://www.blacks.co.uk/cgi-bin/blacks.filereader?40b5e03a0751bc6e273fd43a3b40068b+EN/products/9231203

£35 quid though.

Field & Trek on Baker St are currently selling 15L Lower Alpine sacks with 2.5L platypus systems already in them at a knockdown price of £25 (against RRP £45) - just bought three of them for my friends and I.

PS - they're Lowe Alpine Wadis http://www.adventureshop.co.uk/lowealpine/wadi_15.htm

Thanks for all the help people