My radiators on the first floor do not work until I run some hot water?

We have a problem with our Worcester Bosch “Heatslave 26/32 RS0 GB oil Combi” boiler, which provides hot water for the under floor heating on the ground floor and conventional panel radiators on the first floor.
A typical scenario would be: Heating comes on 06:30 providing heat to entire ground floor plus one bedroom and the shower room radiators. 07:30 both radiators turned off, ground floor heating left on all day. At about 16:30 turn on bedroom radiator (thermostatic valve) and shower room radiator valve (non-thermostatic) but there is no feed to the two radiators until we run a hot tap and draw a quantity of hot water.

The boiler is serviced annually. A 3 port valve in the boiler was replaced 5 months ago.
This problem appears to have progressed from intermittent to regular.