I usually find that if I speak to a sub- contractor with whom I have a quality issue it gets sorted.
Mind you, I don't always go for the cheapest quote. I know my sub-contractors very well- some for over 15 years now. I'll stick work their way instead of Joe Bloggs who's cheaper because I know the job will be correct and on time.
Who was it that said "you can have it cheap, fast or correct... chose any two".
You should have rejected the parts at "Goods In" inspection. You should not have to specify "no clamping marks", it's obvious and legally inherent in the part description. For instance, if the part was delivered to your door covered in rust you would be justified in rejecting it even though the drawing didn't say "no rust allowed". It's inherently implied.
Don't pay them. I'm assuming you pay on 30 days from receipt of invoice? If you've sent the cheque already then cancel it, it'll cost you about 25 quid to do so. Please, please tell me you didn't pay cash up front for these?
I love deadlines- I like the whooshing sound they make as they fly by.