I would suggest getting 3 valuations of your property carried out. One from a large Estate Agents such as Foxtons, a medium size firm and a local firm. Get a quotation of selling fees from each Estate Agent and this will also give you three valuations to work off. You can then advise if you have been given a better offer elsewhere and can open up the field of negotiation.
If you need any advise on a mortgage for your next purchase, i am a fee free mortgage broker based in central London, able to access the whole of the mortgage market. Happy to give break down of costs and borrowing amounts!