I've posted on this before, had the same problem which I solved (after some advice from Steve Parker) by having a Van Dieman formula Ford center exhaust flange welded onto my centre primaries. The problem with this joint is that it doesn't have a center bolt to hold it against the head between the two siamesed exhaust ports, and the standard flange is too thin, hence the warping away from the mating surface when hot which can cause the blowing gaskets. The Van Deiman part is at least twice as thick as the standard Caterham Header flange, and since getting this fitted have never had a reccurance of the problem. I can't remember the name of the company that did the work, only that they reside in the Croydon area and do lots of stuff for exotic engines and racing exhausts. Give Steve Parker a ring. His ads in Low Flying every month.