Citroen C2 central lock

Hello out there, hope there are some clever Citroen and Peugeot mechanics who can help!

I have an old quick citroen , specifically a Citroen c2 1.6 vts where the central lock teases.
I have tried to "restart" the bsi box, and resync the key, after which my key could operate the central locking.

But after a short time no contact between the key-bsi again.

Brand new battery in key, key is flawless and tested!
Have googled that the bsi box can find itself going into "sleep" after which of course it cannot receive a signal..?

Would prefer not to have to replace bsi, far too great of a cost :(

Some ideas for diagnosis, after a loclsmith comes and breaks the car open (door lock doesn't work either) ;)