Okay then, had a bit of a faff after work, the internals are looking a bit sorry for themselves. It looks as though the keyway itself is worn, resulting in the key not retracting the wafers enough to get past the lugs inside the barrel.
The first pic shows the cylinder with key out, and as you'd expect the wafers are all standing proud and ready to block any attempt to turn the lock.
Second picture is with the key in, and as you can see the wafers at the bottom are not fully retracting. Thi, I'm sure, is my problem. The video shows how much movement there is when wiggling the key and how it affects the position of the wafers.
Question is, what's the cure?
Some say file/sand/grind them so they are flush when the kiy is inserted, but will that make it too easy to turn the lock without the correct key - or indeed, any key? If I take the seat lock out will it be the same as the ignition lock in terms of lugs at the top and bottom where it engages?