VSD and harmonic mitigation requirement

Ultimately this is a matter between your customer and their DNO (or National Grid if directly connected).  The DNO will care about what appears at the interface between the site and their network so what else is connected on the site will have an effect, as will the electrical "distance" to other customers and the impedance (e.g. prospective short circuit capability) of the supply.

I suspect that the DNO will use ENA EREC G5 as a starting point but this is a discussion that needs to be had with the customer who in turn should involve the DNO if it has a material effect at their interface.

Indeed,  and if you dont have one, Dcode website has an on-line  a copy ENA_EREC_G5_Issue_5__2020_.pdf

On page 21, is indeed  '5%' listed as a THD planning level for 400V 3 phase ...  lower levels are generally used at higher voltage distro. 
I'd expect an application  for an uncorrected  35% THD load on the network to be laughed off stage. 

Mike.

Indeed,  and if you dont have one, Dcode website has an on-line  a copy ENA_EREC_G5_Issue_5__2020_.pdf

On page 21, is indeed  '5%' listed as a THD planning level for 400V 3 phase ...  lower levels are generally used at higher voltage distro. 
I'd expect an application  for an uncorrected  35% THD load on the network to be laughed off stage. 

Mike.