tetratec ex400 or ex600 end of.
How much you thinking? Shelaghface has a ex700 for sale for £50 iirc?
http/www.ebay.co.uk/itm/TETRATEC-TETRA-TEC-EX600-FISH-TANK-EXTERNAL-FILTER-/200602897308?pt=UK_Pet_Supplies_Fish&hash=item2eb4dd4b9c
a 105 fluval would be a good choice too
http/www.ebay.co.uk/itm/FLUVAL-105-EXTERNAL-aquarium-FILTER-boxed-instructions-/200648541091?pt=UK_Pet_Supplies_Fish&hash=item2eb795c3a3
lol. Matt had one for sale a while back, but it went... eventually