seems the are waving the labour, as @ £588 for the valve, they arnt at a loss! unless its really special on the ST lump, can't see it really being more than a £200 part!
I'd be asking if and why the diagnoses lead them to belive it was this, and that changing has cured fuck all, that they then put your old one back on, as you have no need to spend that cash on replacing a working part.
That's the plan tomorrow when I make a surprise visit.