I think I have resolved the heating issues with SHY

In short, I changed all the hoses, flush gallons of clean water through, put in new water pump, shook the radiator warmly by the throat to empty the sludge out and then set off to check the heating.
Still the same, toasty and warm whilst idling, half a mile at 40mph cool and getting colder

So, it is my considered opinion that the heater matrix is bunged up somewhere and that the fresh air intake provides more fresh air than the meagre amount of hot water can heat.
The answer ???? FINGERLESS GLOVES



Thanks all for your help..... see you soon !!