The following IIF expression isn't working for me:
IIF(Fields!Number.Value BETWEEN 1 AND 1000, Fields!Number.Value, "all ok")
Basically I want to list the value if it falls within 1 and 1000 and show 'all ok' if it doesn't.
Please can you advise where I've gone wrong as this is throwing an error.
Thanks
SSRS does not support BETWEEN as far as I can tell.

Just use equivalent >= and <= checks:
=IIf(Fields!Number.Value >= 1 and Fields!Number.Value <= 1000
, Fields!Number.Value
, "All OK")