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"Research has shown in the past that just a quarter of those clients unhappy with their service were informed of first and second-tier complaints-handling procedures"

What is this "research" from the past? What did they ask clients? Were these clients who voiced any unhappiness to their solicitor but were not informed (again) about the complaints systems? Or were they simply asked "and were you told how to complain?" - which is simply a test of their powers of recollection from when their matter was being dealt with (as we don't even know how long post-representation this "research" was carried out).
If a client wants to complain, they'll tell you. Or they'll look through the papers they've received from you during the course of the matter and find out there.

Simply asking a researchee "have you ever had a solicitor act for you and can you remember being told how to complain about them?" is meaningless.