From the NATS AIP website, Coventry's fire cover is as follows.
Mon-Fri: RFF Category 2-4 available during published aerodrome hours, subject to aircraft type.
Sat, Sun: RFF Category 2 available during published aerodrome hours.
Up to RFF Category 6 available by prior arrangement during published aerodrome hours
For some administrative reason, CVT never offer Fire Cat 5.
The Cat 2-4 on weekdays means Cat 4 as a basis but may be downgraded for a time to allow staff to do other things when Cat 4 is not required by traffic.
I hope this answers the last incomprehensible question