Hi . wonder if the right fan was fitted .have you tried to get whoever fitted it back?
If its discontinued can you still get parts for it .
Check the flue isn't blocked with soot and the heat exchanger is clean . make sure enough air is getting to the boiler . clean the spark electrodes . the oil is not burning completely. Either not enough air or not burning correctly.
Hi . if satisfied with your answer can you give a positive rating , thank you