IIUC, the check would simply be
Month ( Get ( CurrentDate ) ) < 4
Makes no sense here. The parameter to the month function has to be a date. If you use a number, that number gets converted to a date--which in this case would be very different from the current date.
I had been working on the assumption that 'Month (3)' was putting 'March' into the equation as it all seemed to work up until now. I think it was more by luck that judgement that it did until the year changed...
Making the change to erolst's answer has worked a treat.
Thank you both for coming back to me again!