Dropping in a bit of copper tubing should take only a few minutes, from memory there is a smaller breather pipe with a 4-5mm internal diameter, just cable tied this out of the way, it won't affect the cooling system as nothing flows through it.
The small dia pipe is the 'bypass' pipe, you must block this off before you tie it out of the way, you only need to replace the metal bypass unit on the heater 'flow' pipe with copper tube, the plastic bit on the return pipe is just a one way valve and can be left in place.
I didn't do the copper tube fix, I just took off the 'metal' bypass unit from the flow pipe, in one end you can see a copper coloured flat metal valve, I just drilled a 6mm hole through it flushed out the swarf and put it back with the small bypass tube left in place to take some flow away from the matrix, I guess this might keep the matrix temperature down a bit and reduce the risk of it blowing......?
I managed to do it without losing any coolant, (well only a few drops) I took the header tank off and dropped it down just below the level of heater pipes then caught the coolant from the disconnected pipes in a plastic tub, not much at all really, then poured it all back in when I'd finished.
