Your absolutely right Garyj, but it shows that even a small number as above can be effective

Its all about building the data - unfortunately there's no other way, but time does fly. Even if you have only been going for a little while, it can be relatively simple getting mobile numbers.
For instance on a leaflet put a mobile number on and get them to text you if they want to take advantage of an offer i.e. a code or something, and then you can build this very quickly. Or on your website, text for a quote etc. Put these in to your database and you can market them independently from your normal clients.
At the same time you can naturally build up clients in the normal way, the building blocks of the future.