bad workmanship, bad bill?

[farmersmarket]

I had my boiler serviced at beginning of auturm and after 10 days we tried to get heating on but did not work. I contacted plumber by email (have copies) and phone but after a week with no reply I called another plumber who fixed it. First plumber had not tested heating during servicing (As on his worksheet) or run 15 mins test needed by manual but now threatening legal action if I don't pay his bill. I have offered parts but said I don't want to pay labour as it was not done properly. Whats my legal position and should I just pay up even though basically carp job? Thanks