Dilemma: Still being paid from old job

[Myalias]

I used to have a job working for a very large international company. It was just a basic unskilled customer services role.

I worked there on a fixed term contract for 9 months which ended in December, they've since paid me direct into my account for another two months wages.

Has anyone been in this situation before, if so how did it end up. I was happy keeping quiet when they paid me for an extra one months pay but now I'm a bit concerned.

I'm in the UK and the money totals ~£2k if that makes a difference. Also the team that I worked in is being disbanded at the end of this month so there's a chance it may all blow over in a months time leaving me £3k richer.

What should I do?