"Can I hold on to that money until such time as it is payable or do I have to furnish it tomorrow to complete the sale?"
Yes you can hold on to the money. However, when it is due, you have to pay it over anyway. My experience is that 99% of the time the client forgets about the stamp duty, expects the solicitor to remind them not once but several times, rarely produces it on time and then expects the solicitor to work miracles to make sure that penalties are not charged.
Most solicitors like to turn the work over quickly, get deeds stamped and registered and returned to client or lender as soon as possible.
mf