yes I don't know the level of cover etc and taking the op on face value a quid would be to expensive. Assuming your supplier does what they should for digi monitoring its fine.
you can also make it print events in real time. I have used that in the past on custom software. But as you say no reverse commands. I'm not good enough to use the mircotech over serial option
its probably an upgrade to gsm signalling. But there are other options to stop the 0844 number being used. but if there are 0844 numbers on your bill originating from the alarm then it appears it os working. It's not a sky telephone line is it?