Craig, your Astra has a petrol engine, right? My turbo Talon (petrol) does the same thing as your Astra at any cruising speed. It reads zero boost, no matter how fast I'm cruising. But that is how petrol turbo engines are supposed to behave, because they are throttled.
OTOH, it is perfectly normal to see some boost at all times on unthrottled diesels, especially with a very small turbo or with a high weight-to-horsepower ratio like your Passat. A little bit of boost actually improves the thermal efficiency of a diesel, and IMO is not something you should try to defeat.
If you are looking to improve your fuel economy there are several things that can be done, but turning off the boost is not one of them. First, keep in mind that every country's MoT is most interested in cutting diesel emissions, so your engine is both injection-retarded and choked off at the exhaust. Fixing these issues and intercooling the intake charge will do wonders for your fuel economy without attracting undue attention from the authorities.
Of course, the ultimate injection solution is probably Giles' treatment, that might be a bit of work from Scotland, but if you follow the work pioneered by fspGTD and SMOG, you can see huge improvements yourself for the cost of a little time. There is a thread in the FAQ section detailing SMOG's refinements, however, I don't have a direct link to fspGTD's thread on retiming the pump and changing the governor springs, but you can search for it in the IDI section, or if we are lucky, someone will chime in with the direct link. Also, any muffler shop can build and install a turbo-back 2.5"-3" exhaust that will largely eliminate all that dreadful backpressure. Finally, add an intercooler to extract compressor and engine bay heat from the intake charge.
Think performance stuff for your Astra. Pretty much anything that adds power to a turbo'd petrol engine will do the same for a diesel. The idea that these are old tractor engines that don't respond to hot-rodding is so...20th century!
Enjoy the ride!
