I've had these or similar specimens phone me in the last week (after a long gap), despite blocking their number they just keep coming back with a different number.

Indeed - the scamming business must be doing well these days. What's amazing is how easy they find it just to keep getting new phone numbers. I believe there used to be a reporting service for such things, but it may have gone.

There was a 'local' firm to me in North Herts about 3 years or so ago that actually came round to my housing development to 'survey' it (on spec, without permission [it is private land]), who I saw, asked them what they were doing and they came up with similar speil about getting cavity wall insulation. Apparently a fellow 'resident director' on the development had just been scammed by them into getting 'replacement' blow cavity insulation.

Whilst there have indeed been instances of new build properties (our were then 15 years old) not having the cavity walls and lofts insulated (a good few cases listed on Watchdog, etc and involving major building firms), these cowboys were saying all the properties were likely to be like that. If it was, the owners would soon know, given their heating bills would be huge and they'd suffer from significant cold spot areas.

For some amusement (I was bored), I let them go through their entire sales patois before giving them the bad news of what I did for a living (Building Services Mechanical Engineer). I never did hear from or see them around the development again...

I keep thinking about one of those True-call gadgets or whatever they''re called but I might miss my Indian chappy who seems particularly concerned my broadband has been attacked and I might get a virus. I've explained I've been jabbed for Covid but he still keeps phoning.