Maybe.
If you buy 100% of the property:
If the property appreciates, then you gain the full amount it increases.
If it depreciates ( like they are now) then you take the full loss' yourself.
If you only buy 40 %:
When the property appreciates you make money but not as much as if you owned the full property.
if the property price depreciates then you have limited exposure to the losses as you only own 40% of the property.
The other thing to think about is that the cost to buy the remaining 60% at the moment may be, for example, £60k. In 2 or 3 years time when you want to buy the other 60%, depending on how the market has changed in that time, the remaing 60% might be £40k or £80k etc etc.....
you will need to pay rent on the 60% of the property that the housing association own. Make sure the total payment of the rent plus your mortgage does not equate to what it would cost you to buy 100% of the property.