Personally, my first year I didn't bother with professional indemnity ins. I found this more for the advice side of things for the likes of lawyers, doctors and the like, those who mainly give advice professionally. What a PAT inspector does is mainly offer a service with some advice to the customer, not to the extent of a doctor where they rely on their word.
And it was bldy expensive!
However, now in my third year, I've found Trade Direct Insurance to be quite competitive as they offer public liability with professional indemnity ins in one package, you also get your tools covered in the van during the day.
I think it was around £110 for £5 million public (not sure on prof ind).
Just a word of warning tho, public liability doesn't cover you if you break, blow up something that your working on. This is another type of insurance that is alot more expensive called efficacy insurance, but you need about 2 years of trading before you can get this. (if it's worth it? I haven't got it).
It really depends on the amount of advice your going to give, I have it now because I also advise and report on their Health & Safety surrounding their business.
Hope this helps?