As said, cutting or blocking the small pipe will have no effect, the problem with these is that there is a small thermostat in the metal bit that gets tired and shuts off the flow to the matrix at too low a temp, the black plastic bit just has a one way valve in it. If you look down the end of the metal bit (the end with the flange) you will see a flat metal 'sort of ' plug, just put the unit in a vice and drill an 8mm hole through the centre of that, its very soft metal so very easy (careful though, its a bit wobbly). Flush all the swarf out of it then put the whole lot back (make sure the thin bypass pipe is not full of smeg first and make sure the black plastic valve is clear). It will no longer operate as intended but it fits and saves messing with copper pipe, also you still get some bypass flow which does not go through the matrix so the matrix might just run a little cooler. Make sure you get the black bit back in as it came out, if you get it wrong the one way valve will be against the flow and you'll get no heat.
