Right, finally had a go at this again because my car was knocking like hell when I drove it today, worse than ever.
Jacked car up and removed wheel then disconnected the upper joint of the drop link to de-tension the ARB. Unloaded, the ARB moved easily from side to side. I pull the ARB as far as it would go towards me and then roughly measured the gap between the collar on the ARB and the face of the ARB bush using some wire probe things. It was around 4mm in my case. After hunting around for some plastic roughly 4mm thick I hit on some 110mm sewer pipe, available at any DIY store. From this I cut a 38mm diameter disc using a hole saw and then cut a smaller hole in this disc of 22mm diameter (ARB diameter). I cut a section out of the resulting ring leaving a gap of maybe 14mm so it clipped nicely over the ARB without coming off. Finally I chamfered the edges of the cut bit to help it slide in-between the ARB collar and bush.
Then the fun part of trying to slot it into place, not very easy. In the end I managed to use some long nosed pliers to manoeuvre it into the space behind the ARB on the drivers side and wedge it in far enough to hold, then used a decent lever to force it home at which point it clicked into place.
And the result after a test drive .... total silence, knocking gone !
I can only assume that all the noise actually comes from the ARB banging from side to side because even unloaded I couldn't get the bar to move radially in the bushes. Having said that I'm guessing the bushes are worn which is why the bar slides side to side so easily. Are the bushes in fact bonded to the bar when new and once this bond fails the knocking starts ?
Just need to see how long it lasts but this fix is pretty much a freebie so can't complain if lasts only a few months.
And thanks to Powercut68 for pointing it out, excellent bit of fettling !