Well, after having a couple of decentred versions of this lens in the autumn and sent them back, I thought . . . . third time (or fourth, not certain) lucky, and bought another one. This morning before work I finally got down to the task of properly checking all focal lengths at all apertures.
Bingo! It's good - sharp, not bad even at the corners wide open - hooray! So when I took the dog out for a walk this afternoon after work, I took it, and resolved to relax and enjoy it.
After half an hour it stopped focusing . . . . Now it just makes a little noise and you know it's all over, sometimes if you reattach it then it'll focus again for a moment or two . . . sometimes it won't.
Oh well, back to warehousexpress when I get home at the weekend.
all the best