As matters stand, your estates will pass to your son in law. by all means up date the Will as you have suggested.
As for contesting the Will, there is absolutely no point in claiming the Will is invalid since were it to be so, the estate would pass on intestacy to your grandchildren. The only other potential attack is a claim under the INheritance (Provision for Family and Dependants) Act 1975. Unless these distant relatives can show that they were some way being maintained by you, they don't have a cat in hell's chance.
Have you, however, considered leaving your estates to your grandchildren? If you leave to your son in law, there is the possibility that he remarries and passes the estate to his new wife. Why not just leave directly to your grandkids?