Will probably be around what terence says, because it is more than likely that the earthing will need upgrading as well as changing the consumer unit,
How old is the wiring ? the main earth eires to water gas etc are now 10mm, years ago it was 6mm the best thing is to contact a local sparky and get him to have a quick look round first, most of us don't charge for telling customers the easiest and cheapest way to do things, most of us are pretty honest, go onto the nic,napit or eca site and put in a search for local sparky, just enter the first part of your postcode, good luck, Ray