Just for Clarity, I need the IF INSIDE the DSUM. I have written it the other way around to demonstrate but i don't think will work as i wish to add the figures from either field

You could use IIF with DSum.
You could put something like this in the Control Source of an unbound field.

=IIf(Not IsNull([ID]), IIf([NetIssued]<=0, DSum("NetWritten", "TableName"), DSum("NetIssued", "TableName")), 0)

In my example I also check for an ID so the DSum doesn't run on new records.

Thanks. I've removed the IDCheck temporarily while building but is a great idea when all is done.

I'm still getting an error when running this. Also, being that you've put the DSUMs inside of the IFF, will it provide one figure for both fields?

Text_WYTDCIMS = IIf([NetIssuedAMT] <= 0, DSum("NetWrittenAMT", "SalesALLQ"), DSum("NetIssuedAMT", "SalesALLQ"))

I am assuming from the error message that your form is unbound. Access has no idea what NetIssuedAMT is. The field will need to be looked up first so you can check if it is 0 or not. DLookup

Found a simpler way around it! Thanks Mate