In my 5 years ordering from them I have had 3 issues,
1. Damaged crystal out of the packet - replaced FOC with next order
2. Incorrect gaskets shipped - replaced with next order.
3. Barrel complete did not include the main spring (I asked through H&S before ordering if it did and Luke said yes) - Had to then buy spring separately and wait until I had sufficient other items to warrant postage.
A MAJOR issue is that if something is wrong or incorrect you have to wait until your next order to get it.
Yeah, that by itself is pretty ****ty. They should provide replacement parts immediately, especially given the nature of watchmaking. Not doing so puts entire projects and jobs on hold until you shell out more money, solely because re-shipping (which I imagine in your case is pretty cheap considering you're located in the UK, and shipping to the US only cost me ~$10 dollars an order) eats into their profit margins on a sale.
It's just bad business practices all around. I'd like to note, that I've ordered from them a handful of times without any problem whatsoever (besides my orders deciding not to show up). However, I'm not blind to how horrible of company they are when it comes to all around customer satisfaction, both on the retail consumer and business consumer fronts. The only reason I continue to order from them is due to them having one of the largest inventories of parts, often at the cheapest prices around. I also know that I'm offered some protection by my bank, as if they do ever try to screw me over like they do to so many others, I'll surely be able to charge back the transaction, in no small part due to the horrible reputation surrounding the company's lack of refunds which would be discovered with a simple google search.