Notify your bank that you’ll be making a big payment for a car, tell them to release payment there and then without going through the checks, if they decide to check it you’ll need to then do the following, all pre the sale BTW.
You and him drive to your Bank, do the transfer in front of both you/him and the clerk, get the reference on the transfer made as the reg number and if possible in the notes (depends on what CRM your bank uses) add in there his name, the car, the deposit paid and balance.
You have the bottom of the V5, get him to hand the keys over in the bank, drive him home.
It’s that simple.
The transfer should take no more than 15mins, if it’s more then yes you may have to answer a few questions about the transfer but you’ve already noted all the information above so that will/should curtail any delays.
If I were you I would not transfer any money pre sale, dealer or not.