Megaflow/Boiler problem

[ianhewitson]

Hello and Merry Christmas,
Would appreciatte any advice about a hot water problem.

Megaflow tank with a Travis perkins 'Icon' boiler both about 7 years old
The boiler supplies underfloor heating downstairs with no problems.
If we activate the hot water or normal radiator heating upstairs the honeywell timer shows F5 fault . The boiler continues with the underfloor heating requirement but no hot water.
Have made sure system vented
If its an overheat problem i could understand if the boiler was running for a while but will also show same symptoms from cold.
Have tried playing about with lever on 2 port valve with no joy.
Any advice appreciatted.

Regards