Claire,
Some britparts ( or as Womble rightly calls them sh1tparts ) are awful its true but they do offer the complete range of parts, they are cheap and are readily available so some plus points there.
The key is to decide whether the part you need is a critical part or not and if it is, dont use britparts.
I have used them and when we first got our S3, I replaced the complete exhaust system for about the price of a takeway, admittedly I DID have to hit it with a hammer a good few times to get it all to fit together but once in, it worked fine - that was 3 years ago and its lasted well although we had to replace the back box but that bit gets all the spray and mud from the back wheels ( especially when you get it stuck in a field and have to leave it for a few days until you can get enough people together to get it out ! ).
I bought a window seal kit for both doors from britpart and they were also fine - installed with no hammers required at all !!
Happy to help....