I looked into this briefly some years ago.
To be a Pilot Inspector (or whatever they call it) you must hold a valid ATPL and medical and relevant experience to their requirements.
They try to keep the staff experience level high an wide-ranging. It's pointless everyone being an expert on 1 type for eg.
The pilot inspectors do keep current on their types by having professional arrangements with many companies - who are delighted to have them by the way.
Good civil service pay and pension schemes.