WO site states the P-FLAT68III.
Although it reads as a flattener only, not reducer, well it does on the page for it but reads as a reducer/flattener on the ZS126 page. Also reads as £500-600.
Cannot see the 126 as an option for the others.
Does it have to be a reducer?
If not I would look into adaptor rings to attach something like a more standard flattener from WO, one that costs less basically. See what FLO have to offer as they work with Astro Engineering and they seem to have an assortment of thread adaptors.
The Flattener 7A says Z126 and reducers by 0.8, at least in the blurb on the the page:
Just still in the £600-600 area I expect.
Will say the WO site is a mess. At a guess 2/3 is out of stock or discontinued. So digging out the relevant items is tortuous.