Think I should add my full 2 cents, mostly because I'm bored but also because I feel strongly on consumer rights and what exactly that means, what people SHOULD be expected to be entitled to, what they shouldn't and what is just plain crazy.
Every site will have teething problems from time to time, servers go down, orders processed by third party companies miscommunicate or orders are lost in the system completely and it's not as simple as just clicking a few buttons and the order goes ahead, it takes weeks of emails sometimes; and when orders are coming in thick and fast, and there are already problems, its understandable that it can get backed up pretty quickly.
As for emails they can and do go missing into thin air sometimes, I have sent emails that never got to the recipent for some reason and had the same happen to me and I rarely use email as it is, nevermind a company that must have a big email traffic rate. This can also be as a result of spam or spam filters: personally I get between 30 and 100 spam emails a day and the more I block the more I get. If I have no internet for a week or so I can come back to 700-1000 new emails at times and you try fishing through those to find genuine mail? I'm not saying this is the problem, it's probably not that bad, but it could well be a factor in emails going missing and being unanswered.
Take sfr for example: their main business comes from the US and if you watch newtib youll see just how many people are ordering from them, and lets be honest UK orders will prbably take a back seat when they start having issues as they need to keep their main business base happiest. It's not an excuse but its to be expected. If PC is as international as Matt says then maybe there could be a reason things get backed up at times. I dont think it justifies some of the waiting times but how often have you thought "I REALLY need to do this urgently..." and then somehow lost track of it completely and done every other minor thing instead. Again not an excuse but I think the idea may be that you either have to be patient or persistent and eventually it will get resolved, with due apologies hopefuly lol.
As for fitment issues, thats not REALLY the fault of PC, as mentioned that is due to the manufacturer and fitting issues should be aimed at them and not PC, if you get nowhere with the manufacturer THEN you should contact PC and ask them to sort out a refund. Modification parts are never going to all bolt straight on, this goes for body kits and performance parts, again this is not the fault of PC but the manufacturer, but its the gamble you take and you should often factor in fitting costs even on parts that are expected to bolt straight on. Where they don't fit and it would be not cost effective to have them fitted professionally, then they should be returned and a replacement found. If you chose not to send for a refund and attempt to either modify them yourself or pay masses of money to have them fitted then this is your own choice, I'm sure if you consulted PC first you would be advised to return the item instead. You do this off your own back and so you are no longer in a position to moan as there was a simpler option and you chose to do it a different way.
To sum up: problems happen, it's how you deal with it that matters. You should always factor in extended waiting times for modification parts, especially those ordered through a third party: it's not always the order companies fault (I had this problem with my europerformance order - it was the supplier's fault and it took AGES but I couldn't fault EP as they were helpful throughout). Also the bigger the mod the more you should factor into the cost of fitting, especially when buying a cheaper alternative (apparently this is very true for headers lol).