Just thought I'd pop in and put your minds at rest - I managed to get the reset combo working in the end. Now hands up who hasn't been losing sleep over this.
I didn't realise you had to hold down all three keys for a few seconds before it would reboot. I thought a quick flick of the wrist would be enough but obviously not. It looks like I'll have to keep searching for the envelope marked 'Amiga knowledge' in the deep, dark recesses of my mind.