From past experience this is alld ependant upon the contract agreement with the parties involved.

Generally in the 50/50 schemes the other party being a housing association stays as a silent partner and has it written into the lease agreement that the tenant will be responsible for upkeep of the property of which includes the testing and servicing of gas boilers etc.

However a debate could be entered into on whether they still hold a responsibility in ensuring the tenant has had these carried out.

So without seeing the lease agreement etc it is hard to say.

An area where it might be best to contact HSE for definitive advice perhaps.
Useful info at http://www.hse.gov.uk/gas/landlords/index.htm