Hello Roderick,
Pricing things is a little like trying to price a house - costs vary from area to area.
Way forward:-
Check sump price on line :- EBay, Parts in Motion etc. etc. As you probably know, parts aren’t cheap anymore.
The £428 - forget VAT, we’ve all got to pay it! - doesn’t surprise me if it works out at a 2 hr. job.
Labour ( this is where we don’t know your local rates) is likely to be c£120/ hr., so you have over half your cost there.
Perhaps you could let us know the sump (ribbed aluminium?) price Roderick, once you have had some quotes.
Issue with your current sump? Thread damage perhaps? If so, considered Helicoil?
Kind regards,
Gareth.