Was assured by the engineer that the socket was their equipment so wouldn't be charged!

I found this so it looks like you are OK.

'The socket itself is openreaches responsibility, but any faceplates fitted I think are your responsibility.'

Yeah, that's always been a bit of a grey area you can never get a definite answer on. The way I see it the NTE's standard faceplate for a 5a or 5b, the ADSL2000, aka Fat Boy and SSFP are our responsibility but any third party faceplate a customers fits or an iPlate isn't and, well, gonna cost a few quid.

If a punter has allowed damp to ingress unchecked into Openreach's equipment, then I would expect them to raise charges via the CP for that.