Several months ago I treated my TF to a new distibutor. 25Ds are no longer available so I bought a 45D from the BeeHive. The result was an engine that would start OK but ran very rough when accelerated. I refitted the 25D and things returned to normal. Recently I mentioned the problem to an MGB owner who had been a professional motor mechanic before retiring. He had come across this before and said that the problem was due to the tight fit between the points and the pivot. This was not a problem with Lucas points with the red insulator but was with the blue insulator by Ignition Components. Sure enough the points on my new 45D are blue. I hope this can save someone else the amount of head-scratching it caused me.

Thanks for that David, I have just checked to make sure my spare points are red - just in case.