Hilary,
Sorry for my long time reply - I was so tired... I needed sleep. AND guess what, got up this morning and did exactly the same as I did yesterday and now it works.
xdXDocument:GetDOM("For Gasils Products List")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW/d:Surface_x0020_Area[../d:Title = current()/../my:cmbRingDrierDailyProductTarget]
Compare this line with what you wrote and you were spot on (as I would expect)
Sometime its best to just walk away for a while!.
Thanks Hilary, as always, take care.
Mike