[G8BBZ]

Hi Stu,
You don't say if you have any knowledge of electronics or access to any test equipment. Maybe you could let us know.
If you haven't already done so, it's worth checking 3 fuses - 2 located on the rear of the RA17 and the one in the mains plug. You will at least need some sort of testmeter to do this (or a supply of spare fuses). While you are looking at the rear panel check that there is a wire link between the two terminals marked HT1 and HT2. Make sure the set is switched off at this point or you might get a nasty shock from the voltage on these terminals.
If the fuses seem OK, then switch the set on, move the service switch to the "Man" position and give it 5 minutes to warm up. Remove the top cover from the set and look at each of the valves to make sure that you can see the red glow of the valve heater inside th valve. You may need to temporarily remove the screening can where fitted to see this and it can be easier in a darkened room than in full daylight. All the valves should have a glowing heater inside, any valve without a working heater will need to be replaced. If all the valves are OK, and if you have a testmeter, you can check the voltage on the HT1/HT2 terminals mentioned above. This should be about +250v measured with respect to chassis.
Perhaps you could do this and report back.
cheers
Peter G8BBZ