Thanks for info rog747 and confirming the worst. I have just checked in online and had been allocated seats three rows behind where I had paid. Not ideal but at least still slightly forward of the wing. I am going to ask at the counter t'row about the return leg as the system won't tell me what it has done. Seat Guru did not know what aircraft it is, so could not show me any warnings.
The checkin page did not offer to send me the boarding cards by email. Said I could print them online but no option. Half an hour later, I checked my App to see that the boarding pass was there. It invited me to checkin. It also invited me to add my FF number. Given that it haws been there since before the App I was surprised but checked it was correct - it then said, 'FF number already entered for this flight! After another 10 minutes it agreed that I was checked in and had the boarding pass. Their IT systems are down to their usual standard. Which match their mgmt.