How easy / quick is it to drain a HGV water separator? (UK, Iveco Stralis)

I think I derped today and put some adblue in the diesel tank.

Pump kept cutting out, was told to use another pump, 2 mins later the water contamination light comes on.

Tomorrow I have to take it in first thing to be sorted, and wondered how difficult / time consuming it might be to sort. I'm hoping its quick job on the ramps...