mate has the GM low pressure turbo diesel in a mk3 astra.
ok the engine is a bit rattly, but it has NO power below 3K. pump timing has been done professionally. glow plugs are good (starts quite easily). compression seems OK (starter struggles unless battery is reasonably good, 'even' cranking - all 4 cylinders at the same pressure by the sounds / feel of it)
can it be the cat is blocked? the previous engine had gone bang in a big way, would this have blocked the cat and casuse this?
after 3K its better, but not much.
does it need the cat for the MOT? (M reg IIRC) is it worth punching a steel bat through it to clear it?