Sorry - I take it all back! Musket is quite correct.
CAP 168 says....
4 Temporary Depletion of RFFS
4.1 Provision of RFFS to the Category set out in paragraph 3.4 is a mandatory
requirement. However, there may be circumstances when a part of the facility is temporarily unavailable due to an unforeseen circumstance e.g. an in-service mechanical failure of a vehicle or piece of equipment or sudden illness of a member of staff. Immediate action should be taken to reinstate facilities whilst considering whether landings and take-off by aircraft required to use a licensed aerodrome should be restricted.
4.2 At aerodromes of RFF Category Special and 1 fixed wing aircraft, temporary depletion shall not be permitted.
4.3 At aerodromes of RFF Category 2 and 3 fixed wing aircraft, the minimum Category of RFFS required during temporary depletion shall be no less than RFF Category 1.
4.4 At aerodromes of other categories (RFFS) during temporary depletion, the Category of RFFS shall not be less than the equivalent of two Categories below that of the RFF category according to the size of aeroplanes expecting to use the aerodrome (Table 8.1).
4.5 If any depletion is significant enough to warrant a restriction of aeroplane movements then the temporary level of RFFS stated in terms of specific RFF Category, should be immediately promulgated by NOTAM and radio. The policy regarding the maximum duration of temporary depletion should be considered in advance by each licensee and be published in the Aerodrome Manual. Generally, temporary depletion should not last more than twelve hours overnight at an aerodrome when few movements are expected or two hours at any aerodrome during peak traffic periods.
4.6 Exceptions to the above should be made for emergency landings, and for occasions when, in the pilot’s opinion, a diversion or hold may introduce a more significant hazard.
4.7 Licensees should consider developing contingency plans to limit the need for temporary depletion of the promulgated level of services. This may involve, for example, a preventative maintenance plan to ensure the mechanical efficiency of equipment and vehicles, arrangements to cover unplanned leave and absence of its minimum level of RFF personnel, etc. Licensees should consider the provision of reserve facilities to limit the need for temporary depletion.
This is not my experience but I just do what I'm told.
Having looked all this up, I'm not surprised it causes considerable discussion from the pilot's perspective. What it seems to say is that the RFFS cover may be two categories below that which is appropriate for the aircraft and it may be decided (by the aerodrome operator) not to mention it to the pilot.
Of course, it also says that the aerodrome operator should consider limiting availability in such circumstances. And this from a regulatory document? It's good that we all know where we stand!
So, now that you've piqued my interest, how does the aircraft operators' insurance cope with a take-off or landing two categories below what the books say should be there? And how good is the pilot's defence, should he/she need one, that no-one mentioned the depletion of RFFS cover?