the old scheme didnt work very well, and I dont know if it has changed. see above
Cost £40 000 ( yes that old ) - you could buy 25% - 50% etc and get a mortgage on that and on the remainder - pay rent. The idea being that the mortgaged part made money for you....and you paid less 'rent' ( dead money ) but still paid a mortgage as well.
and in practice people got locked into a part ownership scheme and cd nt get out to another house with 100% ownership altho that was NOT the goal when it was started.
To sell - you had to "step up" to 100% ( oo-er!) - with the party I was assisting - we stepped up to 100% and then stayed and didnt move.
Be careful about giving the deposit to the helpee - it may be counted as his savings when it comes to housing benefit - AND on selling - his capital also bad news for benefits ( as in you dont get any)
Check Dyer v Dyer
https://en.wikipedia.org/wiki/Dyer_v_Dyer
which I used to devastatingly effect when he sold
( I cd clearly show not by chance I had lent the money so it was never his)
erm and we didnt use that system again
Instead - - I have supplied the deposit and paid part of the mortgage. - and insisted a room is sublet to generate income to pay the mortgage ( now NOT tax deductible)
Bit of a technical answer here
and if you try "oo I cant make my way foo your hieroglyphs as usual PP yeah" - then - - - you will lose money