A lot of firms, both big and small, happily carry out the basic maintenance regime in BS5839 even if it's closed or a managed one they aren't an agent for. It just gets very difficult when it comes to faults, additions, C&E programming etc.
For any Gent addressable system it is very important to use an integrator as although the systems are very versatile to many uses and configurations it's very easy to mess them up - they are a beast of their own and it is very easy to loose all your programming by a simple error in how you approach the job, even with addressable experience.
I would strongly advise based on the extent of your system you go direct to a Gent approved integrator, the few quid you may save by leaving the basic PPM to a non approved contractor is nothing compared to the damage and disruption they could cause!