Well, if they've gone to G1/4, then this link (which you posted)
http://www.componentsuk.co.uk/catalo...roducts_id=306
should work fine - at least in the sense of fitting - no idea how dependent that block is on flow, and whether you've going to have enough to make it happy.
The issue isn't how much thermal energy water can carry (it can carry an amazing lot) but sometimes blocks are designed assuming some minimum flow. If you fall below that flow, the heat doesn't get effectively from the flock to the water.
Might as well try it and see...