Thanks vampirequeen, I had realised that was why you calculated at £7.50, and I think you have a good point.
However, I will be working to a lower amount for the following reasons:
I don't want to step too far out of line from others locally - this will only be the second 'job' he has picked up, and I don't want to raise expectations too high! I wouldn't take that too far, but it is a consideration.
His next job step will almost certainly be glass collecting / pot washing at the pub, and will be £5 an hour.
Actually I think he is learning the skill of doing a job to expectations (and I take the point about not paying him until he gets back, and may re-think that)
My own sense is to keep it a bit lower this time, and see how it goes, but I thank you, in the spirit of this forum for making the point.