If you were only pulling the tank, why were you doing things with the valves? Sounds like a bit more than just pulling.
That starts to get complex. If you have no written contract, what have you been doing in the past loads? What was your accepted role - so even though it wasn’t written, what did they expect you to do and did you fulfil it?
Is this the whole or part of the bill?
Now is the time to use those free 30 min solicitor sessions. Or make a one off full and final payment without obligation and hope it goes away