Thanks again to John Rumm and Set Square
After trying all conceivable permutations of hoses on radiators, pressurising using mains water etc my final inspiration came Sunday morning. On the strength that the blockage had moved, I hoped that it might have moved as far as the "low flow" rad valve. So I opened the valve and "broggled" through the valve and down the pipe with a tie wrap. Successive rodding operations and some spraying of the bedroom wall resulted in full flow being resumed!
The blockage was found to be caused by a build up of black granular material, rather like finely crushed coal. This was not the normal thick liquid sludge flushed from the rads.
Is this debris from the boiler, if so what? and does this signify the end of life for the boiler?
TIA
Phil