In the meantime, after fixing it, sort of, with gaffer tape as the parts of the switch are basically falling apart, it is now OK.
But I have been onto Citroen C3 websites both sides of the channel and it appears it is a bit of a common thing.
When I gently pulled on the back end of it I could see all sorts of bits of it coming out, spring, contacts etc. So it is fairly obvious that the plastic that should have held it all together has given way/bruck off/got lost.
As it is a particularly pointless bit of the electronix stuff, namely to warn you if you have been stoopid enough to start it up without the bonnet properly closed and also, and more to the point, it is part of the alarm system so if anyone has your bonnet up trying to knick it, the alarum goes ooff!!
So as getting another one will be a nightmare I am asking the C3 fora if there is any reason why I can't just "jump" the contacts in the nearest connector to simulate it being "on" to shut the system up.
How the flip did we all manage when there was no electrical system to tell us if we had closed the bonnet properly or not?
I do resent being patronised by a car I am driving!