I'm back from a few days away and thought I'd give you an update:
2 more calling cards were left after my last post, so I did call the RSPCA number in the end. It was their Inspectors & Enforcement department, and apparently someone saw a cat vomiting outside my house. The operator I spoke to asked if it was mine? I said no, never had a pet in the 10+ years that I've lived here. She (the operator) said thanks for calling anyway, and that it was probably either somebody else's cat that was in my garden or the whistle-blower got the wrong address. I have fairly good neighbours, so I would be mortified if it was one of them. I think they all know that I'm pet-less (no cat flap either) so I reckon it was a postie / deliveries driver or someone else relatively unknown to me.