I'm often around Stocksfield / Mickley way so not far from Hexham at all.
Service at AAS was a mixed bag; the workmanship is great and they have an excellent reputation, particularly with full systems for V8 muscle cars. As such, they have a long waiting list, went in once and got quoted 3 weeks to the build date (about 18 months ago) and when I went in for my latest exhaust I was looking at 9 weeks but managed to get in after 6 because they had a half day empty so they could fit the backbox build in as it was a small job.
They have a very American view on things too - all about tone, not about volume. I had to reaffirm many times that I knew what I was getting into and in the end AAS built the exhaust to my desired spec but without a receipt, basically stating they take no responsibility whatsoever if I get pulled over the police, which is only fair imo, they don't need people dragging them through the dirt if they get pulled because their cars are too loud. I do respect the fact that Stew (the guy you will deal with if you go there) was only looking out for me really, as I do know a few lads who have thought they want it really loud and then after a week the reality sets in and they want it quieter. Just have to stand your ground and let them know you know what you are after! If you are looking for a full catback system though, they won't work out all that much cheaper than a branded system, they charge about £340 for a catback iirc, and last time I looked you could get a Buddy Club Spec III for £500ish if you looked in the right places, and the bonus of not having a waiting list. On the plus side, you can choose the volume, size of the centre silencer (if you want one at all) and the look exactly as you want it. The downside is the waiting! Keep in mind that the Buddy Club B pipes are allegedly quite low hanging even on non-lowered cars and very prone to speed bump knocks.
A friend of mine got a Powerflow exhaust for his Fiesta ST from the place on Scotswood road I believe, they let him specify the tip so he got the look he wanted, but iirc they use pre-designed templates, so basically two Powerflow'd identical cars, from workshops at opposite ends of the country would sound the same. His ST did not sound amazing, it was not all that loud and while it did pop and bang a bit, it was more of a bassy flub than anything else. I can't remember what he paid but it was pretty cheap though. He is onto his second ST this time around and either going for a Milltek or for a real custom.
EDIT: Just to reiterate in case I came off too negative, I am 100% happy with the exhaust AAS made me, for the money I paid it's absolutely fantastic, and their resistance to building super loud exhausts is to stop people getting something they can't handle. As long as you state that no matter how high the end volume level may be, its what you really want, they will build it. I would definitely use them again and recommend them to friends.