it all depends on your point of view. he has allready fault finded the unit. and told you its the fan motor.
personally i dont see how this is possible, however that was not your question.

a fan motor can be rewound and balanced for 150 quid.
if the boiler is old as stated, i wouldnt repair it if it was in my house, i would install a newer more effiecent boiler.

it all depends on how much money you have, and how much your installer knows you have.

sounds like he thinks youve got a bit !!

good luck
shaun