Sorry not to have updated this thread with the final solution I used after so long.

I am now fully 4G Broadband using EE & 1pmobile sim, it works like a dream for me, so much more reliable than the landline ever was.

As for the alarm dialler my solution was an ESP8266 Ardiuno WIFI module (about £5) and a subscription to an IP to SMS service (about £10 once).

The software took a while to write and debug and the whole thing fits inside the alarm controller case and gets it's power from there too so no external connections at all.

Thank you all for your helpful suggestions.