Many thanks for the words that go with the sketches on the Oleson site.
I screwed up the courage last night and did the job. As you predicted, the escapement was gummed up and needed the Ronsonol treatment.
All went well and I now have a set of working slow speeds again - the 1 sec setting is maybe a shade long, but I won't be hand-holding at that speed and it may provide a little reciprocity correction ;-)
Having done the job, I would say that the IIIf is actually quite easy to work on and the better fit and finish helps as you have to get it back right for things to work - the only problem I had was getting the vertical lever back into the escapement rod correctly (eyesight problem).
Thanks to the others who replied for thier thoughts too.
I think the next job is to figure out how to improve the contrast in the rf window without investing in a new mirror
Can't wait to screw a lens on and try this one out!