The two batteries for my key fob (VW Golf) were £2.40, from B&Q I think it was, two or three years on the fob is still working OK. The originals from new (new car) lasted four or five years so still some time to go yet and I do have one of the two original batteries, if any good I could use that one next time.
Good that you’ve got it all sorted now, despite further ‘slight’ problems also now remedied.