Obviously, if you do not feel competent to do the tests it may be worth seeking external help, maybe from a specialist company.

That being said. The 2005 Control of Noise at Work Regulations state that a sound level meter should meet “at least Class 2 of BS EN 61672-1:2003 (the current instrumentation standard), or at least Type 2 of BS EN 60804:2001 (the former standard)”

I would caveat that by pointing out that if you were to appear in a court case it is likely they would want the more accurate readings from a class 1, so for higher risk work it is worth investing in better equipment.

If you do a google search you will find all price ranges. RS components (no affiliation with me) for example do a decent class 2 one for around £100.

As a side note, some phone apps (whilst not adhering to the standards) have been tested, scientifically, and been found to be surprisingly accurate., assuming you calibrate them first. I use one to identify problem areas, which I then highlight to the peron carrying out our noise surveys.

Since the regulations were published the standard referenced was updated and is currently BS EN 61672-1:2013

I dont think it possible to produce a decent sound meter that does not cost a fair bit - The sound meter we had when i took over my current role was far two complicated none of the team new how to use its features, so I sent my collegue off on a training course - then we invested in a new meter like the one he was trained on. Has not got some of the features we had before - but we dont use then, and its samller and lighter to use, and we are getting much better results.