Worcester 26cdi lockout

[simpson31southwales]

hi i have a Worcester 26cdi in december it had a new Expansion Vessel fitted air switch and air pipes after this the cobi was working 100%
however i have a new problem

when the combis cold it runs fine hot water and CH however when the combi reaches its correct temp and then shuts down it runs until
it needs to fire up again at which time i get the lockout if i reset it
it wont fire up untill its cold down again

i dont think its the overheat switch kicking in as i run the combi on the central heating at number 3 and it doesnt get that hot i have tested its hotest running flat out it runs much much hotter and dosnt cut out when i switch heating off and back on again it wont fire up untill its cooled down

i tested the overheat switch when cold and when hot and i get the same reading from it but i dont know if it was hot enought to give a diffrent reading?

i have tested the ch sensor and its fine and the hot water sensor

the flue was checked and the fan etc all fine

also the combi has been flushed out when the Expansion Vessel was changed

i would like to say sorry for the poor english / spelling i am very dislexic