Using VLOOKUP & External sheets - file size explodes !

[gtg]

I've written a spreadsheet that makes heavy use of the VLOOKUP function to reference an external Excel spreadsheet.

I've discovered, that sometimes (not always) OOCalc copies the referenced spreadsheet sheet into my OOCalc document as a hidden sheet. As the referenced sheet is nearly 20MB, this causes my document to explode in size & for OO to slow right down.

However, it doesn't always seem to do this.

Can anyone explain what's going on here ? Is there a way to stop the referenced sheets being copied into my document ? (even if there is a slight performance hit)

Thanks.