Am I liable to pay for amenity area maintenance charges if the amenity area plan wasn't included in my deeds?

We bought a new build property in 2010 which had a green space amenity area as part of the planning agreement. We are being chased for fees for maintenance by the company owning this green space. Our deeds have a covenant for paying maintenance fees, with the clause "(and shown in green on the plan annexed hereto)". The only plan shows our plot (marked in red) and a small area of the land we are being billed for. None of the land is marked in green. The Maintenance company say another plan should have been included, and we should pay, but I believe that we should not be liable as there is no amenity area in the deeds we signed?