Why has my 52reg Vauxhall Combo 1.7 Di lost its power??

Why has my 52reg Vauxhall Combo lost its power?. Its done about 48,000 miles, its been serviced regularly at a well known garage and I've looked after it. But when I start going up a hill it just loses its power, even if its empty. When I pull out of a junction I have to rev the nuts out of it or I dont move anywhere. It dosen't appear to be the turbo, what about the injector assembly?. Any thoughts?.