Topic

Well I understood that Del wants to solve the problem of boot light and all the time I am speaking of this.

The switch which is operated by boot barrel with the key is just momentary switch which provides earth connection to the alarm box to say that key is used for opening the boot.

Yes, got it now thank you. Initially I thought I was seeking a switch that I had not found somewhere around the hinges of the boot.
Indeed yesterday when trying to fault find the problem I did "tweak" that curly spring a little and unknown to me at the time it solved the problem although I think that the 1/4" spade terminal immediately above that also needed a little "exercise" to improve the electrical contact. The spring in question is presumably passing current through the round peg (also circled in yellow) and other metallic components in the boot lock itself.
Does that imply that in order to complete the circuit and continuity to ground that the other spade terminal pointing to the left in the images ~ but not circled completes the circuit with an additional wire (black I think)?

I'd be interested to know the significance of the bit circled in red. Is that the component that engages with the plastic cover and retains that cover in place?

Yes, other spade termimal is there to complete the circuit with ground.

Red circled part you do not have on your lock. That is lock from 85th anniverssary model and is used for remote boot oppening.

If the lock you say comes from the 85th does that mean that fitting one of those locks to my car would make the fitting of remote boot opening easier?

It would make fitting an electrical actuator easier but only marginally. You are now familiar with the lock mech’ & it’s attend tendrils, the easiest actuator to fit is those which sell as aftermarket central locking systems; these give a powerful on axis pull.
Locate the centre of the boden cable from the lock barrel & arrange to pull against that.
I find that it’s the snatch at the end of the actuation which trips the lock mech’.
M

A remote boot release is a DIY job that can be made with commonly available parts for around £ 15 . No need to seek out a rare and costly new mechanism.
I found a generic central locking actuator has enough pulling capacity to do the job. Take it's power from the + feed to the boot light (recommended best practice is to add it's own in line fuse) and run a pair of wires into the cabin for the release button. Make sure the actuator comes supplied with a length of rod as this is just right to bend into shape to make your linkage. Arrange this so it pulls on the same release lever at the back of the lock mechanism that the linkage from the keyhole does.