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Honeywell timer and motorized valve (V4073), Towerstat RS room thermostat.
When I switch on cental heating nothing happens. The lights are on on the timer but light not on room thermostat. "Experts" at B&Q and Plumb Center both suggested it was the motorised valve so I changed that but no effect. There are no volts on thermostat, so is it the timer or a loose connection in the feed to the thermostat ?

Honeywell web site gives standard wiring diagrams I would think your on Y plan. If central heating selected then room thermostat should have power and room sat comes before valve so any indicator lights should work no matter how valve is set. But the valves are not easy to understand the diagram shows one switch where in fact there are three switches inside the valve so when measuring it is not always as expected. The switches are all very small both in programmer and in valve and even if correct 5 amp fuse is used any fault can damage other items not really a DIY job.