There's a couple of ways to look at this.
1. Are they invoicing you for say £100 and you're only paying them £50?
2. Are they only invoicing you for the £50 and that's what you pay them?
I would have thought that if you got them to do things the second way, then you're in the clear. You'd only be paying them the same as your invoice. When they needed some of the money that you're holding back for them, get them to invoice you for the extra amount that week.
Not too sure about it exactly from a tax point of view but it would be easier for you to keep your books right and theirs.
If you did it as example 1, the taxman may say that if the subbie has earned it and invoiced it, he'd want his cut whether or not he has had all of the cash.