Just to save people going to the CAA website, I have copied the relevant bit below.
TYPE OF LICENCE
3.1 An applicant may be granted a Public Use aerodrome licence or an Ordinary
aerodrome licence. In the case of the former the hours of availability of the
aerodrome must be notified in the United Kingdom Aeronautical Information
Publication (UK AIP) and the aerodrome must be available to all persons on equal
terms and conditions. An Ordinary licence relates only to use of the aerodrome
by the holder of the licence and persons specifically authorised by him. The
holder of an Ordinary licence is not obliged to notify the hours of availability in
the UK AIP but, if he does so, the aerodrome must remain open throughout the
notified hours irrespective of traffic requirements.If the hours are not notified
the availability of the aerodrome and its facilities can be shown in the UK AIP as
‘by arrangement’, but if this is the case then the protection of an Aerodrome
Traffic Zone (ATZ) may not be provided.
I believe that Sywell notifies its operating hours in the AIP (I will check later). If so, it has contravened the ANO and CAP 168.