I'll reproduce the fruits of today's phone calls to the DVLA and HMRC - once I managed to get through the lady on the phone was incredibly helpful, I hope her bosses don't find out as I believe it's a sackable offence at the DVLA :-O
It was helpful to have your registration number as she was able to look it up and compare notes - apparently yours was registered as "agricultural" body type.
A lot of the grey areas she said I should either write a covering letter with the form or visit the local office and get their opinion.
An awful lot of this seems to hinge on whether the vehicle is young enough to have been type approved, as that wipes out half the boxes in one swoop
which I imagine would be the case for pretty much anything green & historic.
Reg mark - blank (they will issue one)
Tax class - (whatever you're going for, Private HGV in my case)
Make / model - As it says on the tin
Type of body / vehicle - Seems fairly free-form, I'll probably put "flatbed" or something.
Wheelplan - As it says on the tin
Colour(s) - Green
Type approval - Not relevant due to age (1986), leave blank
Variant - "
Version - "
Length - "
HC - "
Unladen weight - "
Number of seats - As it says on the tin
Max net power - Not relevant due to age, leave blank.
Permissible maximum towable mass of trailer braked/unbraked - Not relevant, leave blank.
Width - "
NOx - "
Revenue weight - "
Number of standing places - "
Max permissible mass - "
Power/weight ratio - "
Date of original registration - As it says on the tin
Date of UK registration - As it says on the tin
Euro Status directive number - Not relevant, leave blank.
Country vehicle imported from - As it says on the tin
LHD/RHD - If you have to ask...
Type of fuel - Multifuel seems to confuse them (she asked if it was dual fuel like petrol/LPG cars), I'm guessing they'll just go for "Diesel" although the suggestion on HMVF was "heavy oil" instead.
VIN / Chassis number - She said if we can't find one the local office will issue one, we then need to MOT it on that number and then put the V55/5 in to get a reg plate.
Engine number - Fill it in if there is one.
Cylinder capacity - As it says on the tin
CO2 - Not required, leave blank.
Mass in service - "
Particulates - "
CO - "
HC + NOx - "
Trailer weight - "
Stationary sound level - "
Engine speed sound - "
Drive by sound level - "
Year of manufacture - As it says on the tin
There are a few bits about trailer towing on the flip side of the form, but she didn't have those on her computer and said I was best asking the local office what they want to see.
I even had a sensible answer from HMRC on the import duty form VAT414, which was basically "Fill in whatever bits you can & send it in, chances are there's nothing to pay as it's coming from inside the EU"
Hopefully this will be of use to others importing stuff from the EU.