The first clue is the letters on the cuff of the uniform; ARP. This stood for 'Air Raid Precautions' and was an organisation set up just prior to the outbreak of WW2 with the aim of protecting civilians from the dangers of air raids. They were responsible for issuing gas masks, air raid shelters and managing and enforcing the 'blackout'. Volunteer air raid wardens were recruited to do this, and at first I thought that this was what my grandfather had done. However, when I looked for photos of ARP wardens the unforms didn't look quite the same.
Clue number two came from the hat. Wardens had tin hats, but this one looked rather different.
Further investigation lead me to this:
This is the uniform from the St John's Ambulance and you can see my grandfather sporting a uniform very similar to that of 1950. During WW2 The St John's Ambulance volunteers acted as ARP ambulance attendants, and their job was to aid casualties, search for survivors and remove dead bodies.
What struck me from the photo was that despite this gruesome job, they all look quite cheerful and they look impecably smart! No sign of a high visabilty jacket anywhere! My grandfather did take this role quite seriously and he would have done this in evenings, after doing his day job as a postman.