A will can be contested in several ways: it is possible that it does not comply with the formalities for creating a will (perhaps it is not signed or witnessed correctly). It could be forged. Alternatively a will could be challenged on the grounds of undue influence, lack of knowledge and approval, or lack of testamentary capacity.
Even if the will is valid, it may be that the division of the deceased’s assets can be altered by a claim under the Inheritance (Provision for Family and Dependants) Act 1975 or by a constructive trust or proprietary estoppel claim.