If you are looking for a missing will, the first port of call will probably be local solicitors or law society. However, if the firm holding the will has been subject to an intervention by the Solicitors Regulation Authority (SRA), its closed files will be stored by the SRA. The SRA holds over 6 million files and many of these include wills. Once a firm is intervened in, its closed files are sorted and archived by Capita (on behalf of the SRA). If you think the SRA is holding a will, you can contact them by emailing firstname.lastname@example.org or calling 02476 339250.
From the Law Society Library
If the firm you are looking for cannot be found in intervention archives, it may be that they merged with another; the SRA can check firm names from 1993 onwards.
If your last known information pre-dates 1993, then the Law Society Library can research successor firms using the resources they hold, which go back to 1775. Please email them or call 020 7320 5946, with the name of the firm and the last known date they existed.