Ok, just ran a quick test (have to take Mother in Law out shortly to East London), switched it on there was this light buzzing as mentioned earlier coming from the rear quarter, took all the covers off, the oscillator is not running and the buzzing is definitely coming from mains transformer, sounds different to most transformer hums, maybe because this one is fully screened?
Checked main power rails, all present and correct, will probe the primary side of the HV section later today on my return.