ExXB
Many thanks for your reply, It's a couple of years since we visited the UK and I couldn't remember if it was required then. When I made the booking I didn't notice a seniors but when the APIS page rejected the birth dates I went back and tried a pretend booking and sure enough, there was a seniors category.
Guess I'll try phoning tomorrow to avoid any possible hiccups when checking in if it won't let me print out our boarding passes online.