If it is on private land then VOSA have no rights of entry/ cannot dip tanks etc in a target action like you describe above.It will be vosa doing the checking and they are self financing.
It is possible they may decide to target matches just like they target auction mart car parks.
Most competitors will only burn a few drums in a season.
It will be the commercial classes with working tractors that are at most risk as they will use red during the week.
VOSA ended years ago, was replaced with DVSA,If it is on private land then VOSA have no rights of entry/ cannot dip tanks etc in a target action like you describe above.
They can only operate on public roads and some council owned sites.
Take a petrol station forecourt, it is not part of the highway and therefore cannot order you to follow them to a weighbridge if you are parked on the forecourt.
Begs the question “what is agriculture ?” these days.VOSA have got their own agenda! When did ploughing become non-agricultural?