Gas cooker, oven not staying lit

I bought a Flavel cooker in September 2007, cascade 50, either model ECA 5, ECA 5L or ECA 51 (not sure as can only see the model number in the brochure.) My oven has recently stopped working as it lights, but when I let go of the button it goes out. I have been told that the thermocouple needs replacing. I understood this wasn’t either a difficult or time consuming job, (looking on another website), but was told by someone else Flavel cookers have a particularly difficult to reach thermocouple and would therefore cost more money to repair than expected. Please could someone let me know if this is the case. Thank you.