Resolved
So, it appears the seat base was being held down by the hooks designed to stop it moving forward under load. By applying some pressure with my knee, I was able to flex the seat base such that it freed itself from the hooks.
I've attached some photos that I hope show the hooks for anyone interested in this issue/resolution.
IMG_7716 shows the seat base that wouldn't move.
IMG_7719 shows the hook that was holding the seat base down.
IMG_7720 shows the seat having lifted, so you can see the hook (left) and lug (on underside of seat base).
Thanks again.