hi ive got a baxi bermuda back boiler which wont fire up?

Sounds like it could be the pizzo igniter thats faulty. However I suggest you call out a Gas Safe registered contractor if you are in UK as its illegal for anone unqualified without the relevant training to work on a gas appliance.Can I ask when did you last get this gas fire serviced and a safety check carried out? As its dangerous to operate without servicing as carbon deposits can block the flue and you get killed by carbon monoxide poisoning which is odourless a silent killer and very dangerous.

There is a thermocouple next to the pilot light which needs replacing. It is a very thin Copper tube.

its the automatic ignighter that lights the boiler when heat is called for not the manual pizzo ignighter