Niall - I had exactly this discussion with Curly in January after running a Grade 3/4 Theory course in Shropshire.
I'd had a peek at the BCU polo refereeing page but needed to know who could do the practical assessments. Here's Curly's responses:
>A number of G2 can assess G3 & G4.
"If you become a G2 then you will be able to carry out G4 assessments"
>They need to be 'signed off' by the cheif refferee first.
"My criteria for G2s to do G3 assessemnts are flexible, based largely on the ability to be up to date, experienced and able to say "no" to candidates who do not make the standard"
Of the 15 who passed we've only managed two practical assessments so far & we've 15+ people for another 3/4 Theory course, so I'm going for my Grade 2 & see if we can get things moving in the NW Region.
If you know anyone interested in getting their 2/3/4 Theory let me know via polo[at]shrewsburycanoeclub.org and we'll invite them once we have the course dates.