pinky, 4 years ago i was unable to get 100% on shared ownership.
times have moved on tho so it may be possible now.
the mortgage options available in part buy/part rent situations are notoriously rubbish in comparison to a full buy scenario. there are actually very few lenders in the area of the market.
which is crap really when you consider the government is supposed to be pushing keyworker homes etc.
you might find that homebuy might suit you better.
i don't know so much about this option but basically you get find a property on the open market. mortgage 75% of it and take a gov loan for the remaining 25%.
when you sell, 25% of the sale price is repayed to the gov. you get whatever equity you have built up after paying off the mortgage.
this looks like a good option as you don't have to pay rent on the share you don't own - which you do when you part-buy/part-rent.
check out Housing Options for more info.
HTH and Good Luck!
ps: I'm not a mortgage advisor but I own under part-buy/part-rent.