1948 MG TC.What locates the distributer into the engine block and how do you tighten it and stop it wobbling about?

There should be a "U" bracket, with a 13mm bolt in the center, that secures the distributor. It fits around a flange on the base of the distributor. You loosen the bracket and rotate the distributor, to set the timing with a timing light. There is either a slot or a gear on the end of the distributor's shaft, to locate the distributor.