Thanks again for the interest. Here are a few photos to show how it all goes together. Unfortunately, it's starting to look a bit rusty. It's painted with White Knight Rustguard "Hammered Finish" which I would NOT recommend. It looked great when freshly sprayed but is now very brittle and every stone knocks a bit more off. Will have to bite the bullet and repaint with something more durable.
The two pieces that stick out from under the bumper are 65x65x6mm steel. That's the same as most tow hitch receivers so a recovery block will just slot right in on either side.
The current weight of the thing is about 60kg plus the weight of the wheels so not lightweight by any means. You can see that I've started to cut some lightening holes. I'm pretty sure I can remove a fair bit of weight without sacrificing strength. I'll finish the job when I get a new drill. The old one has died, probably from drilling too much steel!
I probably should point out that I'm no engineer. As far as I can tell the whole thing is structurally sound and won't damage the chassis rails or fall off on the road somewhere. So far so good.
After putting the bar on for the first time I found a couple of bonuses. First, the flipglass will still open even with the tyres locked in. Second, two standard jerrys fitted nicely between the swing arms. The weight is taken by the rear step and a couple of octo straps hold them in place. It's all a bit agricultural but it works. If I can get the long range tank sorted I won't need this but it still might be handy for water jerries.