Anyone know how to fix this?

[FiremanT]

As you charge £84 for anything, then changing the BV is going to cost less than a tenner extra. And fewer chances of a call back. HArdley "bigging up" a job

 

[AGAS]

You are very lucky!

I wonder how many plumbers would have just done that if you had called them out?

Probably big up the job to isolate the water supply and replace the float valve.

There should be an isolating valve fitted under the cistern! That's required under the Water regulations!

or you could just do the job properly , anyway he might have to just tighten the blue nut just a little more next time , ye he's really lucky isn't he