I have found references to cleaning out the heater matrix of gunge caused by mixing different types of antifreezes/using additives/not changing antifreeze often enough, using steradent. But I have been unable to find the procedure anywhere, despite searching the back posts going back to start of 2009 and searching the UK Jag forum.

Can anyone help by giving me a link or explaining the process?

I assume it is wise to try to blow the matrix clean with compressed air first, having disconnected it from the general cooling system in the engine bay and then what I would expect is to attach 2 short lengths of hose to each of the inlet/outlet pipes and simply fill one with a strong mix of water and steradent, holding them aloft so the mix has an opportunity to fill the matrix and no airlocks are formed. Leave for a while, then blow through with air again? And repeat until no more bits come out?

I assume it is wise not to allow steradent into the main cooling system, or would this help remove gunge from that as well?

I do not have the facility/equipment to backwash the system under pressure.

I have recently drained under gravity (through the wing nut plug near the bottom of the radiator) the system and topped up with 5 litres of anti freeze, so clearly I did not get more than half the coolant out doing that, but I did not leave it to drain very long and I did not "flush" by leaving a hose running at the top and running the engine to move fluids around. I also did not undo any hoses, as I did not have the clamps or the inclination, the job being done on my sloping driveway without good access to the underside.

I have poor cabin heat and believe it will be the water heater pump (bushes most likely), which I will act upon and also possibly the matrix being partially blocked.

 

You needed to go back further. The procedure was documented in Punch by artist Noel Ford on 4 Feb 1987:

Old man washing his radiator grill with Steradent




Graham

 

I had to look that one up. For the benefit of other ignorant folk, steradent is a denture cleaner like Polident.