Have you tried reverse flushing the matrix?
The other thought that struck me, (although I'm not 100% sure as I have a classic), doesn't the P38 have a variable valve in the heater matrix hoses somewhere? I had a discussion with Wammers about this and I seem to remember looking at the diagram in Rave. If you're not getting return flow from the matrix either it's blocked (which the K seal won't have helped) or the valve is stuck/inop.
In the classic there is constant flow through the matrix even from cold, when mine got gummed up I swapped the hoses over so it was running 'backwards', getting heat now . . ..
A
