Annual Service & Nozzle Change?

I am a tenant in a rented house, landlord hadn't serviced the boiler in nearly 5 years. Finally got his plumber around who tells me the nozzle needs changing, reckons he's 'done the rest' of the service.

We've noticed a marked increase in the amount of oil used over the last year or so and assume something isn't right.

Landlord tells me the nozzle makes no difference to oil usage so I'm wondering what purpose it serves and why change it ?

Appreciate advice on this please :-)