it's totally fair, this is one of the main reasons why people don't want to work on old cars like GTT's anymore (me included!!).
another way of thinking about it: why would a garage take on the ballache of such a job knowing that an old rusty bolt is likely to fail because its old and siezed when they could take on newer cars only, make just as much money without having to worry about rusty old siezed bolts. if a bolt snaps in your cylinder head it'll most likely be because the bolt is old, sized or rusted, probably not because they've done it wrong. if I did a job like this and it took me an extra 2hrs to get the bolt out then I'd be charging for that extra 2 hours !! You have to look at it from the business's point of view sometimes, but I agree its a bit of a grey area
PS, can you tell rob works on un-usual / clasic cars too