On my travels through the interweb I found a registration article talking about this;
https://www.gov.uk/government/
publications/iva-declaration-form-for-seat-belt-anchorage-compliance-iva-19a-and-19b
Comparing it to the IVA manual section 19 I believe that this is where it comes into play;
quote:
1. Evidence of type approval
2. Documentary evidence of testing to the Directive standard
3. Comparison with a type approved vehicle
4. Visual assessment
Just visual, they use a tool to check top mounting height as per manual