So changed the matrix core today and heating is restored just in time for summer 🤣. Was a bit of a fiddly job, about 2 hours in total. Strangely enough, there was no sign of the silica bag in the coolant tank. Possibly removed by a previous owner? Who knows.
I did notice a week or so ago the radiator seems to kick in a bit even though the engine temp sits at 90 degrees. It done it again today after the matrix core had been replaced. Possibly another issue that needs looking at.