Thank you for all of the interesting comments and, having read them, I do feel that I understand the issues better. For what it's worth, my view is that PRP is a useful tool for measuring performance and setting goals for career progression, as long as it utilised along side an appraisal system, which is as fair and transparent as it can be.
However, I do agree that a system of PRP can be open to abuse and/or to 'manage out' individuals whose faces don't fit. Inevitably, there needs to be a degree of trust between the parties carrying out and being subject to appraisal, and the appraisal will be subject to a number of subjective judgments. It is also a consensual process and, done properly, can be helpful to everyone involved.
In principle, though, I don't consider that teachers should be immune from appraisal/PRP because they work for a non profit making organisation. I consider that tax payers are entitled to expect value for money just as shareholders of a company.
The comments on this thread confirm my perception of the teaching profession, ie that there are a large number of extremely dedicated teachers who are doing a brilliant job, but there are some who have lost their enthusiasm and have become a bit 'jobsworth'.
In most other sectors it is expected that staff will take place in activities which are not specifically part of the contract and to work beyond the contracted hours. For those working in my sector,the law, this is completely routine. I am not saying that it is right, just that this is life in the real world.