Thanks again digger58. I borrowed a screen off my brothers' neighbour and it told me no end of useless information (I could read it though). It said my navigation CD was incompatible, my child locks were on, I had an anti-pollution fault (like on my petrol 406 coupe) and so-on. However, the stereo did play without interuption. So, I've re-connected our broken/useless unit and the stereo seems to play music without being disturbed by the 45/60 second SOS issue.
This week I have (hopefully) solved a 50% power from cold issue and a worthless stereo issue. I thought that buying a car with the PSA 2.0 HDi engine would be fairly trouble-free. Is my engine just crap or are both the engine and car crap? I'm not a Fiat fan, but our previous petrol (mk1) Ulysse and Mutipla 110 JTD have both seemed relatively trouble free and reliable even if not that frugal 17 and 34 MPG respectively.