Well DD has managed to miraculously find the perfect implement for the job! A spre polybush about 31mm outer diameter and 15mm inner diameter. Think of it like colestorol - your heart has to work hader in a human circulatory system so its bad, but in the this case we WANT the heart to work harder so it should be ideal!
I'll probably fit it just before the rad inlet, it should be a simple as slipping it in and puttng a hose clip around the outside.
Btw my heater blow was actually fine, DD had just squashed the air outlet duct nearly flat with his steel toecaps! Once I straightened it out it was fine :hysterically_laughi.