In regards to the Law, you have to prove (if anything goes wrong) that you have made a “conscious effort to maintain electrical safety within the workplace” So... It mainly depends on the size of your company, the type of equipment you’ll be testing and the frequency.
It’s always best to air on the side of caution. I found out a few weeks ago in court cases that breach H&S it’s up to the defendant to “Prove beyond all reasonable doubt that they did everything within their powers to prevent a foreseeable risk”
I would say that:
If your providing a professional service you should get your tester calibrated annually.
If you work with industrial / construction equipment you should get your tester calibrated annually.
If you operate & test a small office or other low risk environment it’s recommended BUT I know of many 1000’s of companies that do not calibrate their testers.
You could claim that by doing the Combined Visual & Electrical Safety testing you meet these recommendations, but that would be up to you to prove in court if anything were to go wrong.
These are only my personal views and I am in no way qualified to give legal advice.
Hope this helps....