After the rain this afternoon I had another bash at this heater matrix. I hooked up a pipe and funnel to the intake tube and filled it with drain unblocker.
Left it half hour and hardly any was consumed. Leaving some in the pipe I hooked up the garden hose and pressurized the matrix. Nothing at first but after a few mins I was getting a slow drip from the outlet pipe. Swapped the flow round a few times and it ended up being a slow trickle. Luckily I just walked away as it went boom and this brown crap exploded out of the matrix. It went everywhere, some even hit the wall of the house about 10ft away.
Cleaning the engine bay will be the last job on this car
sxbaz - Assuming this doesn't start leaking, I have a spare heater matrix which you can have