PO Vaults - leaving at xx:58 gives 7 minutes before the yy:05 service arrives at platform 1 from Shkotland. This leaves adequate time to use the station 'facilities' after a few and get downstairs as it arrives to leave at 10 past. Allow an extra minute if leaving from P6 or further afield. I've done this once or twice....
I lived there for over a decade, and walked past it at least twice a day, and never went in!