i seem to suffer from turbo lag
Best way to drive a turbo is of course to keep the revs in mind, - so the turbo is always ready :-)
If, however, you have lost a huge amout of power in general, you might check the EGR circuit. The EGR is a valve, that lets exhaust gas recirculate to the inlet. Sitting in the exhaust, it can become clogged with carbon. One way to free it up is to find an empty road - especially behind you (!) - select 2. gear and go for a while with full throttle, so the govenor keeps the revs at max (4500 or thereabout).
This will probably produce a fairly scary looking cloud behind you (hence the need to empty road :-) ) - but the exhaust should quickly subside and become transparent as normal, and by now the EGR should be reasonable clean. Power - and your smile - restored :-)
On older Espace TD's you could have the turbo-hose fall off, which was *very* apparent from the bellowing sound, and complete lack of power. The modern DCI's don't seem to suffer from this.
/Lennart