BHF the thing arrived this morning ! It has now been converted to a cyclone (or if I have the direction wrong, it will be an anticyclone). I used mdf for the internal baffle and nylon to make an adapter for the vac hose. The swirl is a bit of 32mm FLOPLAST waste plumbing. It seems to work OK, so I have dismantled it to paint the mdf. I have to clean out the vac to check the effectiveness of the cyclone, but when the paint dries I will photograph the bits and post the results.
Cheers !!