Had a querry recently re above and post April changes. Found that both guidance and Zebedee say such an agreement should be with a social landlord or private firm.
The HB (Amendment) Regs 2007, SI 2007/1356 (should you want a read) don't mention any sort of organisation.
Took this up with Shelter /JohnZebedee. Reply from JZ was that when writing the book it was assumed that shared owner would be unlikely with private arrangements and probably that is why the guidance i written this way. If you have a private shared ownership arrangement follow the regs. Any request for a reconsideration/appeal should be succesful (baring other issues of course)
M

The 20th (2007/8) edition of Findlay has the consolidated update and commentary at p257.

The update was effective from 1 October 2007 and its clear that all shared ownership tenancies are covered