So, I can confirm that a 4 into 1 system off a C will fit a 1990 D model without any modifications. Many thanks to Steven Warth for a painless transaction and to Joe Newton for giving up some of his Saturday afternoon to help out.
Am really pleased with the new look and was staggered by how clean the engine was when I took the fairing lowers off. Apart from the knackered old headers there was absolutely no corrosion, rust or anything else to indicate the bike is 17 years old.
Will keep over winter and come spring time will prob be for sale, so keep your eyes peeled