In July 1994 the private detective agency Nationwide Investigations Group conducted an electronic counter-surveillance sweep of my parents' home in London. They checked for radio transmitter devices, and tested the telephone line for attached bugs. They found nothing.
I am afraid that I was unsurprised at their not finding any evidence of covert surveillance. It had been made very clear to me, particularly during 1990-92, that audio, and almost certainly video, surveillance of my parents' home was taking place. But this would not have been made quite so obvious unless the persecutors were confident of their apparatus being undetectable using the technology the police, or a private agency like Nationwide, would be using.
I don't know very much about the surveillance technology that has been used against me, but I understand that devices can be built which switch off on receiving a coded command, and may switch on again after a counter-surveillance sweep has completed; that devices may rapidly alter the frequency of transmission, "frequency-hopping" devices which presumably cannot be detected in a sequential scan of the sort employed by Nationwide; and of course "probe" microphones can be inserted "through-the-wall", although I hesitate to believe our neighbours would permit this.
We paid Nationwide £411.25 (including VAT) for the surveillance sweep, which took them about an hour and a half to complete, using a "Professional 5000 multi-scanner, CCL UHF scanner and Guideline telephone tap detector." As I said above, I don't know very much about these things, so I can't comment on the capabilities or otherwise of this equipment. But clearly the "watchers" are using technology which in 1994 was beyond the detection capabilities of a good private detective agency.