It wasn't sorted, but I wasn't a million miles from the problem. The knocking resumed fairly quickly after my triumphant post above! Today I bought and fitted a new rear mechanism, more in hope than reason. No difference apart from making shifting far lighter and more positive. I decided to give the shed a tidy while the weather was good and in a bag I found a brand new genuineShimano 7 speed freewheel. I cannot recall when or why I got it but never being one to look a gift horse in the mouth I decided to fit it and just see what happens. Several silent miles later with no knocking I am feeling more confident that this time I have cured it. It does mean that I have to use the downtube shifter on friction rather than indexed as it is a 6 speed shifter but that is no problem. It's just like the good old days!