First of all, thanks very much for your help.

I have been thinking about the use of windscreen wiper motors, and come to the conclusion that they're more trouble than they're worth. I think it's better to start with something that works, rather than fighting with something that doesn't from the outset. They have mismatched speeds, they're heavy ( 1kg each+steel box section frame= overweight) and are huge.
Now I've got that out of my system...
Does anybody know of any motors which would do the job, considering that they will need to be quite slow, but with loads of torque?

Thanks