This was the nightmare that had to be tackled. Flow through the long length of tough-flexi pipe had ground to very little indeed causing problems with the heating. No option but to dig it up and replace with 2" for lessened resistance. Chopping the first pipe was the point of no return. After a day of grafting came the big switch on - voila; lots of flow. Thank goodness. I am guessing that the layer of scum on the inside of the pipe, combines with flexi that was at no point 100% straight, was all adding to the resistance. A lesson learnt.