If Des is away doing his day job in theatreland, it could be some time before he gets back.
Bill Bedford used to produce sprung W-irons for 21mm but, when he transferred the production to Eileen's Emporium (also now long defunct) the 21mm irons didn't resurface. They are probably long lost now.
The alternative, I suppose is to start with a P4-compatible W-iron and then graft in a section to widen it.