Ariston Integrated Dishwasher leaking from sump

I have an Aristion A2080 Integrated Dishwasher that I have identified a small leak from a hairline crack from the base of the sump housing.
I’ve got the replacement sump and tried to fit it at the weekend however after removing all the pipes and wiring etc and exposing the sump ring on the inside of the dishwasher I cant get the two parts separated.
I didn’t want to apply too much force…Should I hold the sump ring and twist the housing or vice versa or is there a special tool that I need.
Many thanks