Not an easy one to answer, as even though Parish Councils 'should' follow national guidance, they are perfectly entitled to set there own pay structures.
However, to be an effective Parish Clerk the person is recommended (but not compelled) to hold the SLCCs ILCA and CiLCA Qualifications (some go on to complete the Community Governance courses as well).
The more qualified a Parish CLerk is and the more experience a Parish Clerk is, theoretically, the higher the pay should be, up to the maximum the particular Parish Council is willing to pay (or sometimes as the case may be, be able to afford). I am aware in my own District of a very highly qualified Parish Clerk do their job for the same pay as someone who is unqualified, the reason being is that they are happy to do it. They retired as a CIty Council Clerk and wanted something to keep themselves active and yes they have been publicly offered more and at the same meeting they have publically turned it down.