All equipment requires an annual test, not just hired. If you go into someones house with a portable and their child gets a shock, if your machines been pat tested and regularly serviced chances are you'll come out ok. If it hasn't been you could be in deep ****.
Ignorance is no defence in court.
As self employed we tend to become a bit detached from whats going on in the real world of business. You can soldier on quite nicely, everythings going well, but theres always something to bite you in the arse. Cover it at all times.
If you buy a pat tester make sure you do the appropriate training course or its not worth having. Most of the suppliers offer the training and as far as I'm aware you don't need to be a qualified electrician.