The workbook is shared, yes. Thanks, though, that is in the right area!

I have put it on the local c:\ drive of the BIDS machine I'm developing on, and it works, and Preview shows the data in tablix. The drive that the excel workbook was on was a network drive, internally in work, but, obviously, with the network drive has priveleges that haven't been granted to my local SSRS preview engine account (??).

To clarify, it works in BIDS preview when the workbook is on a local drive of the machine with BIDS on, but won't when the ODBC points to a network drive that has additional security (this is why it connects fine in ODBC admin). When I wrote about "the engine" that drives Preview in BIDS, it looks like I was right that this is different to that in Query Designer, as it is running not under My Profile, but under a local SSRS account i.e. that hasn't been granted access to the network file share?

It should be fine as long as the service account that I deploy to has rights to access the file-share?

I'll report back!