Ideally use someone like Jacko that is well known and recommended.
Failing that pop onto
www.seriesoneturbo.co.uk and ask about, there are a few S1 experts in brum and there is also a welder that did some work on a friends S1 and made a nice job and was cheap for what he did.
SueEd is the person to speak to on there as they are who had the work done by him. (the are in Coventry so not far away).
I imagine that on that site with the S1 peeps that are based in brum, you will get a good list of people who can carry out the work. Just be prepared as sometimes as siad, you get digging and find alot more than expected.
Matt