Cost to balance radiators

We noticed that some of our radiators were not getting hot so a plumber has recommended that he balances them. He has quoted £400 which seems a lot,
we have 15 radiators, each has a cover which he will have to remove, but still, is this expensive?

Many thanks.

400 to balance a system pure and utter pish take and covers usually just lift off brackets
do the basics first .
the rads that are not getting hot turn every other rad off and just have one of the cold ones on force heat through repeat for each one then have all the ones that were cold just on for about 30 mins before turning all the others on .
If you have trv`s it maybe the pins have stuck on valves