It really could depend on a number of things, installing a new CU now they need to comply to 17th eds. Can show other problems within the excisting circuit which will cause the RCDs to trip. Just one common problem due to excessive earth leakage.
Ring around and get some quotes, I live in West Yorkshire and I could charge a price that would not be the same as if say in London or in the north west.
Hi, as KB says it depends on many things but in my area (south Kent/East Deleted)coast £300 plus tax is average plus main earthing upgrade would be approx £40 for cables plus time at £35/hr.
get a couple of quotes from leckies who come & look, DON'T take 'phone guestimate, it will either come high to be sure or will get dearer with extras
Don't forget you must get test sheets & whoever does it MUST be in a Part-P scheme eg NAPIT, ELECSA, BSI, ECA/BRE, or even niceic.
if the 3 quotes are all about the same then thats probably on the money. it does seem fractionally high, I would charge about 4-450 in Dorset but it is probably reflective of the work involved in doing the bonding, which can be considerable. I'm afraid the bonding is an absolute as well. no sensible leccie would touch your installation if the bonding wasn't up to scratch without remedial work.
My preference has always been to go for the middle quote, as they probably aren't skimping but aren't taking the whatsit either!