I don't get that. As there is no way you would have needed new pads with that mileage (and I hope you were not charged for any), so it should have been a simple visual check as part of the 'service'. So quite why it started happening after the 'service' is baffling.
I see Wessex are a HPC dealer which is somewhat alarming when they then want to charge you £150 to cure a brake noise that from your description is certainly "not characteristic" - a term you might like to use in further discussions. I would take it to another dealer - hopefully that is what Nissan will say.