Strange Zen Credit to Bank Account

I randomly checked my bank account this morning, and there's a credit for £3.38 from "ZEN INTERNET LIM" with a reference of "ZEN INTERNET LTD BGC" (Bank Giro Credit".

All good, a couple of extra quid in my pocket, except I haven't been a Zen customer for over 2 years!

I was with them for many years prior, but moved to an altnet as soon as they became available.

Any idea why Zen is sending me a credit (which I don't know why would be due) after so long?