Hi Stuart - our PG 1 explains how to apply for first registration -
Practice guide 1: first registrations - GOV.UK
As you have inherited you will need probate and the executor to transfer the ownership to you as the beneficiary - that makes it a compulsory first registration -
HMLR Guide Page · HM Land Registry
The 15 years you refer to is 15 years and more prior to the date of your parents own purchase
Land registration is complex, designed to protect legal and financial interests in property. There can be significant consequences for any error. Please consider seeking legal representation. Before making an application without legal representation, it is important to
consider the benefits of using a conveyancer.