How can I replace a thermocouple on J25 - 32 MK3?

I would be grateful if some one can tell me how to replace a thermocouple on a J25 - 32 MK3 Modairflow, Johnson and Starley.
The last time it needed replacing I called out an engineer and he charged £60 for 5 - 10 minutes work. At the moment I can not afford to call out any one.
It looked like a very simple process and I would be grateful for assistance with this task.