The current telephone entry system handset seems to be broken, so looking at options to get it replaced. I've emailed my local electricial - Alistair Fraser - to see if he has any experience of such installs. The replacement handsets don't seem too expensive, but it's system dependent to need to get the right handset to avoid blowing up the system! At the same time I've been looking at remote access system, that will allow me to let in a visitor remotely. Quite expensive, but they look like just what I need.