hey i had problem come back error a 77 well this time it was the condensate pipe inside the boiler itself.. undo the front cover and follow the condensate pipe up into the bioler.. it runs on the right hand side goes up to tthe top and pushes on to a a square shaped box. in my case the pipe was bent slightly not allowing the water to drain.. check this pipe is free to drain the water.. this sorted my problem out.. i should imagine its a common fault. it hasnt got much room in the boiler so the pipe can easily get bent..
hope this helps.
hope this helps.