Okay folks - I did the deed at the weekend,it worked, and here's a summary. Note! It worked for me: YMMV.

1. Cleaned off what appeared to be a semi-solid impact adhesive from the pseudo-rubber "push" button- don't lose the centre spring! I used extreme care and the edge of a vegetable knife as a scraper;
2. Mixed a portion of Araldite Rapid,about the size of a filter-cigarette tip;
3. Moved the flash head so that it's vertical;
4. Try a dry run: place the rubber button into it's correct position and make sure no bits stick up. When you're happy,
5. Applied adhesive carefully to outer edges of the button and also carefully to the corresponding metal area on the SB-600 and join the rubber to the metal;

When you're doing this initial lining-up, try and get the lettering that says "push" to be the correct orientation!

Wrap your button and flash gun with Sellotape (clear tape) and leave it for an hour: longer if you can. Depending on the ambient temperature, you may want to clean off any excess.

Hope this helps!

(NB.. Some adhesives seem to based on flammable compounds, and I was reluctant to use these on a device that can generate kilovolts, and sparks)