Thank you so much for your help. Several thousand records, I found a null and some decimal points which didn't show up too well.
All is now OK and feel a bit sheepish:o
I'm having the same problem, with a couple of new small test tables it works ok, but using the main tables and comparing what I think are identical fields it just doesn't.
Did you manage to solve it?
Insert a Text Box in your report and in the Control Source properties of that box type the parameter as you have it in the Query, enclosed in square brackets eg [start date]. Actually, probably better to copy & paste so that you don't make any typing errors!
George