The beast lives again!
Removed air filter, pulled injectors off their stubbs and placed to one side of the frame, removed offending inlet stub. Pushed steel rod with magnet attached down guide, aligned top of valve with magnet, then holding valve stem with forceps, managed (after a lot of swearing) to get tip of valve entered into guide. Rotated engine gently to raise piston enough to keep valve in position, then by hook and by crook and juggling magnet and forceps, gradually eased valve up through guide. Unfortunately, the piston can only be raised a small amount before the slid-over rocker begins to bind, but by then the valve was aligned and so I was able to use the magnet to pull it all the way up.
Lesson (if anyone else is stupid/careless/unlucky enough to suffer the same fate) if you do drop a valve and it is still engaged in the bottom of the guide, leave alone! Don't touch anything until you can devise a magnet on a rod which can be used to hold the valve in position; the hardest part of the rescue is getting the valve stem located back in the guide.
I've had a week's holiday this week and I've spent almost the whole week engaged on engine work. On Monday I adjusted the valve clearances on my Honda Jazz and as anyone who has done so will tell you, it is a hell of a job, involving stripping half the car down just to get to the rocker cover. At least the GT1000 was not as difficult as that. I think that I've earned a beer. Thanks to those who chimed in.