Never heard of the make.
From 2 sources..
"Safegard has to date installed 1,500 systems since April 2010"
"Safegard has to date installed over 3,000 systems since April 2010"
http://www.safegard.org.uk/case-studies/monitored-intruder-systems
http://www.securecomlimited.co.uk/?page_id=1387
Same director too
I can't think of any wireless system that only takes 2 PIR's, I suspect some miscommunication from them.
I'd take comfort in the knowledge you have a couple of sensors, very unusual for a landlord to provide a monitored system, let alone FOC
The contract will be between the Landlord and Security company, if they agree to install an additional detector I doubt they would do if FOC and if they do I would also expect the annual maintenance charge to rise slightly to cover the additional equipment.
Loft access during or indeed to gain access to a premises is very rare ime.
Finally you wont be able to add your own for the reasons above and also it would need to be programmed onto the system.
I bet it's not documented as such, very unlikely anyway.. but still not a leg to stand on, as PJ say's get some costing for additions that they feel comfortable with.