They have to give notice (I assume 4 weeks/1month) to their current landlord so its likely that the tenancy with me will begin around the end-ish of October.
I could maybe hold off and select another tenant who can move in sooner.
The couple have said that they'd be willing to pay a reduced fee to me to keep the property for them, so my question is, what is reasonable to charge them?
I'd be more concerned to ensure you don't end up waiting and then they don't take up the tenancy. So if they signed the letting agreement and paid a deposit, then maybe half a months rent? Or just a token?
People who are giving their current landlord proper notice and are willing to pay you a holding fee are tenant's worth having. Certainly they should pay you the deposit now and it's up to you how much you want to charge them. If they don't move in you will then not be at a loss. I'd clarify the moving in date though.
Just to put you in the picture, I spoke to the new tenants and while they serve their month's notice to their current landlord we've agreed that they pay a half months rent as a non refundable 'holding fee'. When their cheque clears I'll hand over the keys and they can start moving their belongings over.
Everyone's happy!