Piet,
As can be seen, there were several contractors involved with these trailers, Arrow, Rubery Owen and Sankey. You have now narrowed it down to Rubery Owen, and it looks like it might have been experimental (?). I have Janes Vehicles & Ground Equipment catalogue for 1983 and the two wheeled trailers are listed, also the same photo as that in the centre of your page, with the walking beam type of twin axles. The text said it was a trial to use two axles, because as the single axle trailer was primarily for large generating sets, they could be difficult to man handle, were top heavy and unstable when set up on soft ground. I can vouch for the manhandling bit, because we used to get the 40kw sets in on these single axle trailers. Also the two axle trailer had a system to raise and lower the suspension for each axle, doing away with the need for a jockey wheel. If your trailer does not have this suspension lift system, I would look carefully, because it still could be a made up conversion. Unless more info can be found.......
Your page photo is hard to read, but I just noticed the pic at the bottom, similar to yours is a 1 tonne capacity !!