Tonight I anticipate making a journey from Northampton, requiring a ticket to Boundary Zone 3. I expect the booking office will be shut and this ticket is not available from ticket vending machines. NRCoC 3 states I should buy a ticket to allow me to commence my journey and then part-exchange it for the desired ticket. But where should I buy to? London Terminals would be too much. Should I seek out the guard on board or pay at the excess window at Euston? They don't tend to come around very often.