The straightforward bit:
You have the right to apply to the Chancery Chambers at the Royal Courts of Justice (or through one of the ten Chancery district registries) for an 'order for sale'. The court would, unless there are exceptional circumstances, normally grant such an order.
Getting a bit more complicated:
Here's the standard format that such an order would take:
http://www.justice.go...f_eps/pd73_apdx_a.pdf
You'll see that the order defines a minimum figure at which the sale must be made, but not a maximum one. However, under Section 8, you'll note that you have the right to further apply for a variation of the order. Such a variation might include a requirement for an independent valuation but you might need a good lawyer to argue your case in that respect!
Chris