bizaar behaviour after removing the stop solenoid from peugeot 204 diesel

Whilst trying to solve a problem with power loss/irregular idle on my
Peugeot 205 diesel I thought perhaps the stop solenoid was partially closing
restricting the fuel supply. I removed the little plunger from the solenoid
and then re-fitted the solenoid. There was no improvement regarding power
loss/irregular idle so I decided the solenoid was not at fault.
Unfortunately when I removed the plunger a little spring pinged out never to
be seen again, I thought rather optimistically that it might only be to
prevent the plunger sticking, so reinstalled the plunger without said
spring.

I started the engine - no problems, then turned off the key to stop the
engine - it kept on running. I suppose this means that the spring helps to
return the plunger to the closed position. I opened the bonnet and pushed
the stop lever on the fuel pump, but to my surprise nothing happened, the
engine just kept on running. I kept it pushed into the stop position for
quite a while but still the engine kept running.

I got back in the car and decided to take for a quick drive up and down the
road forgetting that the keys were in the off position, as I put the car in
reverse the engine stalled. I started the engine again, turned ignition off
(engine still running) and tried putting it in reverse again, just as the
gears are meshing the engine stalls - as if I were turning the ignition off.
It works every time without fail although I can't think why. Could it be to
do with a voltage drop when the reverse lights come on perhaps? I'm going
out to see if turning the headlamps on stops the engine and will post the
results.

I'll just have to find that spring I suppose, but I am quite concerned that
the stop lever on the fuel pump doesn't stop the engine any more, it was
working fine immediately before I removed the solenoid for the first time.

I hope there are some people on this list who have more experience with this
than I do and can shed some light on the situation for me.

Regards
Philip K