“3….in respect of any engineering or other solution which may be devised to deal with the consequences attendant upon the collapse of the Boundary wall, the contribution of the Coopes shall be a rateable proportion of the cost of such solution by reference to the length of that part of the collapsed Boundary Wall which is contiguous with the properties at 41 Orchard Lane and 62 Armstead Road and in any event such contribution shall not exceed the cost of providing a wall capable of retaining land to a depth of four feet along the length of that part of the collapsed wall which is contiguous with the properties at 41 Orchard Lane and 62 Armstead Road.”