Which charging unit? That for the 12V battery or part of the charging system for the traction battery? If the latter, then which part(s) are they quoting to replace? They've already charged you £450 so they should be able to give you full information on the fault(s) they've found.
My concern is that they replace a lot of hardware based on guesswork, find that it doesn't fix things, and then want to charge more to replace something else. If you do go ahead with a fix, what undertaking are they giving that they have correctly diagnosed the fault and their quoted work will fix it?
As always, get second and third quotes if possible. Is the car still driveable and has it any range left in it?