I really feel for you
@Mick, not nice at all to have a vehicle nicked. I have been incredibly lucky- in 35 years of driving I've only ever had a Mk2 Escort pinched- and it was about ready for the heap (In fact its brakes were so scarily bad they abandoned it 100 yds down the road lol!).
My younger bro had a 'modified' Nova a good few years back- we shoe-horned a SRi 130 engine into it and he was worried it would go walkies, so we simply fitted a hidden kill-switch in line with the fuel pump.
Your theory of someone 'in the know' selling the details to undesireables also rang a bell. My Dad (again a ggood number of eons ago) put his Sierra 2.3 diesel (remember them?) in to have its injector pump repaired at a local respected Bosch agent. It was a long and costly job (over 2 weeks and almost £2K IIRC) but the car ran sweet after that. Then, less than a fortnight later it disappeared from his driveway, in an area not exactly renowned for car thefts, and re-appeared 3 weeks later not far from a well-known (dodgy) dealer in diesel cars. Minus its engine & gearbox. Funny eh? Plod were informed of the possible link but weren't interested.
FWIW, I think you have gone down the right route- make it as hard for the feckers as possible. The 'stock' security items- immobiliser, tracker etc. are probably easily over-ridden by someone in the know, whereas fitting your own adds the element of mystery.
I wish you the best of luck getting it back.