Try ringing a few engineering workshops and ask if they have a pipe-bender on the premisis that can also manufacture flanges, if they have then they can do it for you. I know there was a place in Brackley that used to do it (up at the top end of town near the old train-station) but I have feeling they went several years ago.
The last time I had this done was by a place in Bedford (where I now live) but once again they have since moved on. They manufactured a complete V8 exhaust for me which was made out of some serious gauge steel so rot was never going to be an issue! I just gave them the templates and they set to work, I then took the sections home and if they needed tweaking I took them back where they made the necessary adjustments. The olives where the down-pipes met the manifolds were formed out of the steel-pipe by the machine, a really good piece of kit that made it an easy job. At the time I think I paid about £100 which was quite a good deal.
