Assuming we are talking the same parts, 400 quid is really taking the ****
With air/electric impact tools pinions seals 20 minutes each, stat gasket if all 3 bolts come undone again 20 minutes incl fag break
Drive shaft seal again if we are talking the one in the stub axle not much more than an hour maybe a bit longer if bolts wont come undone, if drums may need to remove brake pipe and bleed up that wheel only afterwards, so lets be generous and say 2 hours.
Thats an awful long way from 400 quid
I would speak to them and confirm you are talking about the same parts etc