We had the same sort of problem when we went to XI - your users need to ensure that the locale used for the PC (control panel) is UK, and also ensure that their Webi locale options are both set to ‘English (United Kingdom)’ and that the ‘Use my formatting locale to format the data’ radio button is checked.
Please note that these are report level formula which will never work for this. Edit, apologies, there are two issues one for the prompt and one for the report level format, I’ll get my coat .