My AX dimension has lost all the heat from its heater! It hasn't been all that efficient for a while, but a new water pump seemd to have cured the problem, plenty of heat......for one day! Then it disappeared again. I've tried flushing the system with speedflush to no effect. I can't believe it's the matrix which is blocked, or why would I have got loads of heat one day and none the next? Any ideas?