how do I fix thermocouple on older model Glow worm spacesaver ?

I recently had my boiler serviced however this morning I found the pilot light had gone out . I have tried to relight the pilot light It burns for 20 secs but when I take my finger off the gas supply button the pilot light goes off . I believe this is a thermocouple problem but dont know where this is located on the floor standing boiler and therefore cant see how to replace this Part for old Glow worm boiler 40CF Mk 2 I need to know where is this located and how to replace ?