I worked for a Pye dealer after leaving school. They had hundreds of Hybrid Pye sets out on rental the mechanical tuner sets were getting a bit long in the tooth by then (1977) but we kept them going as decontrolled rentals. Around 1980 they were sold off as they came in ex-rental but the varicap sets were rebuilt at any cost. As Symon has said the black focus assembly sometimes overheated I saw a few where the plastic had really melted I think the VDR went low resistance and glowed red hot. Later ones didn't suffer and the grey plastic was supposed to not ignite. I never heard of one of 'our' sets catching fire, but a local (Deeping) TV shop had a nasty fire in the workshop when a hybrid Pye they had left running on test over the lunch hour went up!
I think my set must have been smouldering away when I switched off and then ignited once off. it was strange that I saw no smoke until I blew the fire out...
Luckily the damage to the back is minimal the back was fitted with some of the GEC type screws with the built in washer which are a little undersize for the slots in the Pye back so they let go without too much of a struggle. I was going to fit some PYE screws with the brown washers as original but now I am thinking I may keep the quick release ones!
I am going to fit the replacement tripler shortly but for now I have carried out the official CES approved modification....
Rich