Further update and in relation to an issue that was being reported by others.
I started getting communication errors reported and when interrogating the alarm panel it was showing 'Jamming' errors (approximately 4 per day) with each being cleared about 90-120 minutes later. I've not been able to determine what is causing these errors.
I do however use allot of Lightwave devices, Somfy blinds as well as Honeywell Evohome radiator controls all of which are communicated over the 433.92 Mhz band along with Visonic sensors. I suspect that its the Honeywell Evohome due to the cold weather activating radiators at odd times of the day/night, but can't prove it (I have looked at the timing of the jamming and it looks pretty random).
Anyhow by changing the Define Panel - 19.Jam Detect to 'Disable', it seems to have stopped the comms errors in the Vera environment, which was in turn was causing the alarm to cease arming because of the Jam condition.