The residuary beneficiaries can next take a Grant of Letters of Administration with Will Annexed. Technically, they will not be the executor, but will be an administrator.
If the named execs don't want to be involved, all you need is a Deed of Renunciation from them - the Probate Registry will then clear them off and issue the Grant to the res ben.
If they cannot be traced, you will need to show the PR that you have done all you can to trace them. First port of call will be Law Soc records department. Assuming they cannot be traced, you then apply under s116 Supreme Court Act 1981 to the PR for a discretionary grant to the res ben.