Many thanks everyone for your replies and suggestions.
However, the problem seems to be solved for the present if not permanently.
The plumber suggested replacing the return valve which he had removed on the midsection of the house, but this time much nearer to the boiler than previously on the original installation 30 years ago.
Apparently a return valve should be as near to our wall-mounted kitchen boiler as possible but had always been in situ on the opposite side of the house under the floorboards of the middle floor.
The new water-pump fitted on this midsection to accommodate the rads in the loft (3rd top level) is more powerful, and was therefore affecting the rads on the mid section. The plumber moved the return valve much nearer to the boiler on this midsection and voila problem solved.
However, the problem seems to be solved for the present if not permanently.
The plumber suggested replacing the return valve which he had removed on the midsection of the house, but this time much nearer to the boiler than previously on the original installation 30 years ago.
Apparently a return valve should be as near to our wall-mounted kitchen boiler as possible but had always been in situ on the opposite side of the house under the floorboards of the middle floor.
The new water-pump fitted on this midsection to accommodate the rads in the loft (3rd top level) is more powerful, and was therefore affecting the rads on the mid section. The plumber moved the return valve much nearer to the boiler on this midsection and voila problem solved.
