I ended up buying a 'proper' front disc, so have gone back to my original plan of converting an old HED to a rear track disc.
I bought one of those SPNK adaptors on (from ebay) but the only way I can get the correct chainline (with the wheel sitting centrally) is to flip the sprocket. I cannot see any reason why I shouldn't do this but thought I would see if anyone else on here thinks otherwise.
As a side note, I wanted one of the old style HED decals to cover a blemish on the disc but could not find one anywhere online. I have drawn the artwork, if anyone else wants one let me know and I'll email it to you: