Many thanks for the responses.

It all concluded happily; yesterday afternoon I had an e-mail from Pentax service, who were doing the repair, to let me know the lens was ready. Pretty swift service!

Needless to say I shot off to collect it and it's now fine, and the quality of the optics, in my opinion, has to be seen to be believed, stunning!

Best regards

I'm not sure whether anyone has reported a problem with the SDM on this lens before, I've trawled the internet a bit and couldn't find anything obvious, so it could be I've got the only example.
It was a week ago that I bought the lens, I was off to a wedding in the U.K. (as a guest - thankfully) and thought this lens would be ideal for getting some candid shots.
I got home and eagerly attached the lens, switched it on, having checked that AF was selected, and, nothing . . .
I tried manually focussing, put my 50-135 on the camera, which was fine, tried the 200 again, nothing . . .
So I gave up, went to the wedding with my trusty 50-135, and on Tuesday took the lens back where it is now undergoing repair.
I'm not too sure why I wasn't given a new lens or offered one, perhaps it has something to do with a lack of consumer protection here in Switzerland.