As suspected flight was overbooked. Was given a boarding pass with SAG on it and no seat number (seat at gate I presume) and sent airside to wait for no shows at the gate. There were 4 of us. Interestingly 2 of them had printed pdf boarding passes so somehow they managed to check in on line and were still on standby. Not sure how that can happen. Anyway as it happens, all was well, there were enough no shows and we got allocated seats at the gate. In sunny England now
Handy to know for future reference so I can start formulating a plan B if it happens again.