I suggest that you call the housing association and tell them about the lender's valuation. Tell them that you are concerned that you are paying too much for the property and ask if they will consider a reduction in the selling price. Offer to provide them with a copy of the valuation.
The Housing Association may be able to give you documentary evidence of recent sales. These will either confirm the lender's valuation (so ask for a price reduction) or confirm the valuation as reasonable (in which case take that to your lender to make the case for an inaccurate valuation by their expert). You can get recent sales info from sites such as upmystreet and right move.
Remember, the difference in valuations is £10,000. If the lenders valuation is correct and you raise the extra £2,500 the house will need to go up in value by £10,000 before it's worth what you paid for it. Less the dreaded stack of fees that go with this horrid process.
Hope that helps.
J