It's known as an Aunty Daisy system in the trade, it sounds like a no bells monitored system fitted by ADT when you enter their monitoring agreement,it's a pay per monthly with a minimum contract.
The code displaying means you have to contact ADT for a five digit anti code.you have two options here:
1 contact ADT and sign up for a monitoring agreement,
2 find an engineer/co that do Aunty daisy takeovers,
bet you didn't know that somewhere out there they are two key holders, meaning they each have a set of keys for your flat.I would suggest ripping it out and change your flat door lock.
Maybe the guy,s on here can offer you better advice they will be better clued up on this than me, we don't do Aunty daisy takeovers because we have had legal implications in the past.
e manno, July 2012