In the end went for the Pentax M 75-150 F4,having seen a nice one at a fair price.
It's not as short as I first planned,but on reflection I should try harder to keep using the 50mm standard lens.
The zoom will help me with portraits and informal shots of the grandkids,plus at the long end should help me with my pictures of merchant ships in the Port down here.
On top of which it's performance is highly recommended in almost every refence I've seen both on RFF and elsewhere.
Thanks again everyone for the helpful comments.