So as a knee jerk reaction I went and bought a Cat 1 Thatcham Alarm/Immobiliser off Ebay.
Upon receiving I see the insurance cert needs to be filled out by a registered Thatchams installer - didn't tell you that in the item description

Anyway I'm going to install myself as it seams pretty simple. Anybody done this before and if so where did you think was the best place to put the alarm unit? The installation instructions say preferably somewhere where interior body work would need to be removed, I've read the dash removal refitting is a weekend job so was hoping for some insight into a slightly easier but still inaccessible location near the dash/ignition/relay/fuse loom. Or am I kidding myself and should accept it is behind the dash somewhere?