Centre has just handed me an inbound B733 requesting an ILS. I am therefore providing an approach control service. So should my callsign be 'XXX Approach', to indicate to the pilot what function I am carrying out, or 'XXX Radar' to indicate I am using surveillance equipment to carry out the function? (After all, Tower and Ground use their callsigns to indicate their function, and do not change to 'Radar' when using ASMI). I also have three training Senecas in the hold. Since I can't use radar to separate them, should I talk to them using the callsign 'XXX Approach'. And I also have a low level military Puma below cover, just made a courtesy call and asked if I can get him Northolt's weather. Effectively he is getting a FIS. Should I now be 'XXX Information', and confuse myself by using three different callsigns at the same time?