citroen picasso petrol pump?

My grandsons picasso will not start, he has been told that if he bangs the petrol tank from under the car while ignition key is turned on it will start.Done this and
car starts, which suggests that petrol pump is faulty.
Main dealer quoted £400 plus. ouch. How can we get to the fuel pump ?.There does not appear to be access from inside the car,does the tank have to be removed for access ?. Is it a hard job for a grandad with a fair amount of common sense, HELP....