Hi Barnacle,
I am guessing this is a 1.6 JTDM (multijet) diesel. Diesels do not need dump valves in the same sense as petrol as you do not have a throttle body that fully closes to control revs (they do have a butterfly on later diesel engines but this is for nice engine shutdown without judder and to partially limit intake for emissions on light throttle). This is probably why it works differently as you describe.
If this has a dump valve it will be used for when the butterfly starts to restrict flow when going from full throttle to partial load to help turbo life but I think only even the ECU thinks the pressure will be over a certain limit.
Have you checked the EGR valve in this instance as they are the biggest issue on every diesel and when they start to go they can be intermittent and normally start to be felt on low revs. When the revs rise and the turbo starts to overcome the recirculated exhaust gases you get performance back.
I know on the 1.9JTDM's (multijets not the earlier ones) there was a new modified gasket between EGR and throttle body to restrict the EGR flow due to similar issues of poor low performance. I did this mod on my 2008 Bravo 1.9JTDM and was a Fiat dealer modification. Its the same for all the Vauxhalls that use the Fiat engines.
If you haven't already checked the EGR I would have a good look, if it is heavily clogged (most likely if never been changed) then cleaning them out doesn't really work but worth a go but replacement is the best way.
There is more possible things that could limit low rev power, but depends if it has a VNT turbo, these can stick if the car doesn't have an Italian tune up every so often