Hi Hollypop,
It's really up to you what you want and what you have to spend.
I had damp in my van so I purchased a £20.00 model of the web.
Gave me an idea of how bad it was, did all the repairs and the
figures dropped.
The next time I had the van seviced the engineer gave me a report
(by way of a diagram) of where he tested and the readings. I followed
him the same day with my meter and found it to be - /+ 1%.
Ok I agree if you want it to buy a newer van then maybe the cheapy is not so good,
but if like me you want to check for damp occasionally then why waste your money?
Hope this helps
Kev