You could compare to similar sized councils/levels of responsibility and see what they pay - but if you look around the variance in salaries for clerks is quite high, and often at town/large parish levels you start to have grounds teams and facilities to manage etc so they vary considerably.
I do know several larger parishes where I live have got in a consultancy to review the size/responsibilities and give a realistic grading structure suggestion (as obviously other roles scale from the clerk). They charge of course but if you are stuck then that might be a good approach, as if you undervalue the role you're of course limiting the people that might apply.