You might be low on refrigerant, take it to a reputable shop and have it checked.

Thanks. I should add my ac blows cold even without that fan running. Dies that make a difference to your recommendation?

Most of the ac places round here, Cleckheaton West Yorkshire I'm not sure are that reputable?

I'm guessing by A/C fan you mean the aux fan. It would help if you included some more information on the vehicle you are working on and maybe the reason you replaced the fan and what kind of fan you replaced it with etc. Maybe try checking/clearing any codes like mentioned in this thread.

There aux fan only runs as needed. The AC won't run at all if the fan fails the startup test. A bidirectional scanner will run the fan at various speeds to test proper operation.

Usually aux fan will turn on as something like 220F if AC is off and 180-200f when ac is on but that's just a generic example. I don't know the rules at all for e53. I do know that I've gone months without the fan kicking on then weeks where It kicks on every day.

E53 has the mechanical fan and the aux only needed to suppliment when that's not enough airflow (eg stuck in traffic and hot out)

Haha I thought they were in the middle of a heat wave over there but London is only 20 C so saying the A/C blows cold doesn't mean much.