Thanks for taking the time to think about my issue. The part number should show what it is, but I've just done a picture to help. Good shout.
A surgical glove finger is a very good idea, thank you. It needs to be thin enough to not impede the movement of the trigger lever, and that would definitely fit the bill. Even a washing up glove would be thinner than the original seal, and so should move freely enough, and should last a little longer.
Here is the photo. Nice and clean now, of course, lol.
Basically, the black box to the left houses a micro switch, and whichever way you move the arm, up, down, forward or backwards, it depresses the plunger on the switch.
The boot is just on the left hand end of the spring steel part of the arm. The big hole above it, is the rear discharge hole, for context. So the grass flies out of there, piles up in the basket, until there is sufficient weight, or pressure, in any direction, to move that arm.
Thanks
Sutty