Are you sure it wasn't a mate of a mate winding you up?

Valid point but I am completely certain it's not a wind-up.

I'd also question why such a place would give a cloned number for incoming calls?

I can't reasonably imagine why either. The only theory I can offer is these are illegal videos and traceability (or lack of) is important.

Either that, or your number is identical to one with a similar area code that *is* a porn shop and someone printed up a bad batch of cards at some point and hasn't realised.

That assumption, and the 'fun' element of...
me: "Oh, you mean the PORN videos?"
caller: "Er, yes"
... is the only reason I've not questioned this before.

What rattled my cage was the fact the caller had already (he claims) ordered/received DVDs via my number.

Something's wrong here I know. I can't figure out how, for example, how the seller made the original sale without me knowing about it.

I know for certain the seller is based in London and so are most (if not all) of his customers, I live in Oxford (about 30 miles away as the crow flies). I wonder if the cellphone provider's topography/geography could be a factor?