Thanks all for your comments and guidance. Yesterday afternoon I had a look on Ebay (before checking with the Forum) and saw a set of x19/124 stalks (NOS) - and despite my earlier comments on NOS decided to go for them. I'm hoping they will do the trick - but even if not I know they are more robust than the CBS ones. The ones I'm going for look very similar to the photo Chris has posted. I'm not after a 100% visual match to the originals and near enough is good enough for me - so long as they work! We will soon see!
Thanks Craig for the wiper motor drawing - I have been pondering over this for a couple of weeks now - and it was by studying this that I realised that it could be the stalk-set that was causing the problem.
Thanks again for all the helpful comments and offers of assistance.

Peter