The NALC and SLCC published a National Agreement on Salaries and Conditions of Service of Local Council Clerks in 2004, which identified four pay bands, according to the scope and complexity of the role. These are described as LC1 to 4. Each pay band was then aligned to a range of points on the NJC payscale. For each band, there is a published profile of expected elements and, unless the Parish Council takes on additional responsibilities or staff to be managed by the Clerk, the band is unlikely to change. Increased hours and modest precept increases do not affect the banding. It is common for Clerks to reach the top of the scale for their particular position and, apart from the annual inflationary increase, their remuneration will not rise any further. It was never the intention that councils in this position should simply re-classify themselves into the next band up to give the Clerk a payrise!
Profile 3 includes elements such as 10-20 staff and a budget of £250,000 to £750,000. If you Google Parish Council Clerk "Profile 3" you'll find the document on many sites.
It's also worth mentioning that the top of each scale includes discretionary points for going above and beyond expectations and that the agreement is only guidance, so a reclassification may not have been the only way to tackle this. The other important consideration is that you are evaluating the job, not the individual, in making this decision, so you will be obliged to offer the same scale to an incoming Clerk when the current postholder leaves.