For catchment delineation you need to develop hydrological models of flow direction using a digital elevation model and ideally a mapped river network.
We use ArcHydro hydrological modelling tools in ArcGIS using 10m elevation model based on LiDAR data and the Detailed River Network as the inputs. More info on the tools and process are here:
From there we use the size criteria set out in the WFD catchment typology to determine where water bodies should end \ catchments should be defined. This is >10km2, >100km2, >1000 km2 and > 10000km2. Where the size theresholds are breached we defined a catchment from the next downstream confluence.
We carried this out across the whole country to create a continuous set of catchments >10km2.
Hope this helps, if not I'm happy to take you through the process in more detail.