Hi Paul
This is a long shot - but it happens infrequently:-
The electrical terminals could be the wrong way around, please check - disconnect the power supply first
Or
You could try taking the float part of the cut out switch off - (the end of the float switch that moves up and down )and turning it around, its usually secured with a circlip take care not to drop the circlip. The electrical polarity could have changed when you put the new transformer on.