BR68F, the licensing conditions booklet states that
7(1) Subject to sub-clause (1a) below, which does not apply to operation via repeaters during transmissions, the Licencee shall transmit the callsign specified in the Validation Document:
During initial ('CQ') calls;
at the beginning and at the end of each period of communication with a licenced amateur and, when the period of communication is longer than 15 minutes, at the end of each interval of 15 minutes;
at the beginning of transmission on a new frequency (whenever the frequency of transmission is changed);
by the same type of transmission that is being used for the communication;
on the same carrier frequency that is being used for the communication.
Sub-clause (a) simply says that when you make a 'CQ' call you should give your callsign. A 'CQ' call is basically an invitation to begin a conversation or contact. Obviously it makes sense to ensure that those people receiving your signal know who is transmitting so they can decide whether or not to answer.
(b) specifies that you must give your callsign when starting and ending the contact and every 15 minutes in between. You could think of it as saying 'hello' and 'goodbye' with your callsign. Note that most of the time amateurs will give their callsign much more often than this, partly as a courtesy to any other stations who may be listening, and partly out of habit. You do not need to give your callsign in every 'over'.
(c) says that if you change your operating frequency, to avoid interference for example, then you must give your callsign again.
(d) and (e) make the rather obvious points that you must give your callsign in the same transmission mode and on the same frequency as the rest of your transmissions.
7(2) At a Temporary Location, the Licencee shall:
(a) use the suffix '/P' with his callsign and give the location every 30 minutes to an accuracy of at least 5km by a generally used identifier (for guidance see note (t) to this booklet)
7(3) When mobile, the licencee shall use the suffix '/M'
The callsign issued to you and given on your Validation Document specifies your main station address. Persons receiving your signals should be made aware when you are not operating from this address. When working telephony (talking) the suffixes are pronounced 'stroke pee', and 'stroke emm', so the station M3BBQ operating from a vehicle would identify himself by saying 'emm three bee bee queue stroke emm'.
Identifying whether to use '/M' or '/P' is not as difficult as many make it appear. A station set up on a hill-top cannot change location while transmitting. It's location is temporarily fixed. A station in a vehicle obviously can. It is the characteristics of the station setup that decides. Thus a station set up in a car is '/M' even if the car is parked in a lay-by, also a hand-held radio is '/M' because its location changes as you walk around.
The notes given in note (t) to the booklet BR68F specify the accepted way of identifying your position to satisfy 7(2A) above. An explanation of the IARU locator system can be found here.
Extracts from the syllabus and BR68F ©Crown copyright Radiocommunications Agency