Series 2 Removing cat flap door handle

Can I have some advice and information please.
I've just repainted my upper tailgate or "cat flap" and I've bought a new door handle and lock.
I thought that replacing it would be an easy job - unscrew the two screws that hold the old handle in place and simply renew the old handle which some kind soul had tried to "jemmy" the lock with a screwdriver or similar.

Things are never easy with a Series. I've unscrewed the screws and they just turn.
Can someone tell me exactly what I do please?
Under the backplate on the inside of the door there appears to be a captive nut protected by something.

Thank ou.

Under the inner handle there is a round spacer which is held in place by a spring. If you push the spacer in you'll see a split pin which holds the inner handle on. If you remove that it will all come apart - be careful not to lose the spring.

Thank you very much for your reply.
Unfortunately the inner handle was missing when I bought the hard top but your post has just given me an idea............
Incidentally I have only just realised that the hard top is from a Series 3 and not a 2/2a so the inner panel is slightly different in having four
securing screws for the locking bar.

I will report back.