a great poser mav

You could be right, b-y

But I have thoroughly enjoyed this thread, mav, even though I can't completely agree your final list.
For instance, "Mission" and "Tension" are identical usages. The double "s" in the first is a function of the preceding vowel, which should have its pronounciation altered if there were only one "s". The bit that gives the "SH2 sound is the "si-".
The same argument is true for "pressure" and "sure", I think. (although I can see other arguments that contradict what I have claimed, in that instance.)

And you CERTAINLY CANNOT be allowed to have "sure" in as an example of both "s" and "su" !!!

Apart from that, it's an excellent list! Many thanks for it.