Just spoke to Nationwide, the guy said that if the money appears in your account on the app, it is cleared and available for you. Apparently it can't be "called back".
Also there's a 10k limit per transaction using fast payment, but he said that they shouldn't be a problem if the buyer did 3 transactions to cover the full amount. A chaps payment can deal with more in one shot but can take a day or so.
Before turning in last night I put the van on Gumtree for 26k, and was instantly offered 22k! so we'll see how it pans out.
Thanks for your advice so far.