Any ideas what this might be?

[Ric2013]

You have to assume that everything that might contain asbestos does contain asbestos until a lab test says otherwise.

I've known personally two people of my own age die from mesothelioma, one a commercial plumber the other a doctor and, trust me on this, it's a nasty way to go. (The doctor was probably exposed as a med student when they had a den/common room in a service corridor beneath the hospital. They shared the corridor with asbestos lagged pipes.)

As I said, I'd want a sample tested to confirm. It's not a huge expense. We're not saying different things (but I agree with you, we need to spell this out to the OP and your approach is exactly that of the HSE: assume everything is asbestos unless there is clear evidence that it is something else).

And yes, this is what I would do. I once had a customer say she wasn't too bothered if I drilled through a bit or Artex. My reply was, okay, but I am bothered! For myself, from the point of view of the law, and from the point of view that I don't want to take asbestos home and into the houses of other customers. Truth be told, I suspect I've already been exposed to asbestos, but that doesn't mean I want to buy any more tickets for the mesothelioma lottery. One ticket can be enough, but the more tickets, the more chance of winning.