Welcome, Nigel. Great to see one more 72nd scaler!
Unfortunately, I have not built Roden's Gotha but if I were to do it, I would definitely look for Part PE set. As for SE5a I have recently built one - you may find my modest build log in Under Construction section. If you have any questions regarding these models or generally 72nd scale, I will be happy to help you. Email or PM is fine.