Tried brazing and to start with it went well but then loads of contamination came out of the cast and I had to give up. As the manifold would be scrap i thought I would try mig welding it instead, to my surprise it worked well and manifold is now repaired and sealed ready for fitting. Normally cast iron cracks when you weld it but I think a combination of very thin cast, short bursts of welding and lots of patience finally paid off. I will hold back until I actually refit it in the morning and see what happens when it is hot but fingers crossed.