Immobiliser problem

Humph I have started taking off plastic trims in preparation for welding new sills. Gone to use it tonight and won't start. Central locking isn't working on the fob but does from the key. Tried disconnecting the battery earth and no charge when reconnected. New battery in the fob and still no joy. Have now disconnected live and earth on battery.
When I took inner kick panel off there is a green box on passenger side is that the alarm brain.

Hi. You have forgotten the model year of your 300Tdi disco, is it manual or auto? I assume that is the vehicle from your other posts.
The only thing disconnecting the battery reconnecting the battery dose is you will have to re synchronise the security system by pressing the fob lock button 4 times.

So you have found the alarm control unit, and which possibly is the reason remote central locking or the engine isn’t starting.
Theres a couple of fuses involved that one may be faulty so just a quick check of those first, I don’t suppose the red led is illuminated on the dash either.