According to the picture and the reviews, this is the HSA-6200 series wireless alarm which sells for £89.00 at Screwfix , £104.99 on Amazon.
I've had one of these for 18 months and it's been faultless. Despite the few mediocre reviews, these are very easy to set up compared to others.
The remote sensors alone (PIRs) cost about £20 on ebay and the system is able to accommodate extra sensors or bell boxes if ever required.
Total bargain. I would definitely buy this if I didn't already have one.