Update: Went back in and they decided it was the button on the handle. However, as it was now outside the 3yrs their initial stance was that I'd have to pay, despite that it relates to a fault they tried to fix under warranty.
A call from RUK CS happened though and we came to an arrangement. If I paid 5% of the parts cost (£10) I could have 12mths labour and parts warranty. I've never been sure of the situation on warranty on parts replaced under warranty tbh, and I even know a legal bod who's had similar wranglings with BMW over such things.