Thanks for all the feedback. I've just had a quote from TextLocal and they also require purchase of a "bundle" - minimum price £4.50 - but in this case they roll over from month to month. I've asked them to confirm whether or not they would ever expire.
I do already use IFTT, which can alert me whenever I'm connected to the internet. This doesn't work so well when I'm abroad, where I can be disconnected from 3G for long periods. IFTT also often introduces a delay, which is not ideal when your home alarm has been triggered. I also have the iSmartAlarm system, which should give me more-or-less immediate alerts, but ONLY when I have data access (again, not so useful when I'm abroad).
The 3G dongle sounds like an interesting idea, but might be slightly beyond my means. I also haven't had great experience of using USB comms devices with my WIndows Server machine. I'll take a closer look at it to see if I can make sense of it...