There is mould in the shower next to the ceiling fan and the roof is concaved, how serious a problem is this?

You have a cold bridge in the area of the ceiling fan. The mould is due to water condensing on the ceiling.This is due to a lack of insulating material in the ceiling and around the fan exit tube.This phenomena is called a cold-bridge and you should get it fixed, as condensation can cause mould and rot in other building structures. If you can access the ceiling over the shower from a loft, then you can insulate from above, if not then the ceiling has to come-down. 200m/m of insulating E.G Rockwool, is a minimum requirement. The concaved ceiling maybe an advanced warning that other structures have already, been effected.