Thanks, al-yeti - I looked at the eBay device, but it doesn't have as many programmable features as either of the two auto-diallers in the original post, and I guess the cost reflects that.
James.wilson - I understand that using GSM is unlikely to be as reliable as landlines, but my thinking was that the auto-dialler in the original post would have one SIM card, and there would be a separate card in the Huawei router which would support additional CCTV (plus a weather station), and provide a back-up warning to an intruder. If the cards were with separate providers, then it's unlikely both would fail at the same time, so fail-safe?
Both the auto-diallers above have several additional features, and I'm not clear why they would need a separate control panel.
I'm just an amateur, and I realise that I'm going to need an installer who's familiar with GSM auto-diallers, to go through the options with us. The property is situated in the BN7 postcode, so would appreciate a professional making contact ( two warehouse/hangars, one needing CCTV/alarm, the other external CCTV monitoring).