Shared Equity - are we stuck???

Hi, we have an apartment on a shared equity scheme from Gladedale aka Ben Bailey homes. We bought the property for £120k (mortgage was for £90k and we now owe approx £83k on the mortgage). We believe the property to now be worth approx £85k. We are looking to maybe selling and are wondering how we go about doing this as we would have to find approx £20k to pay back as part of the scheme...is this correct? If so, are there any feesable options as as you can appreciate £20k is hardly an amount we would have in the bank.

Please advise as we are looking to sell up to hopefully buy a house as we need more room due to a little arrival 6 months ago ;-)