Less performance after new timing belt

Gentlemen, your opinions please.
A mechanic friend of mine has just replaced the timing belt on my Disco 300tdi with a dayco belt and britpart pulleys.
However, now the engine seems to have a little less oomph. I realise that a new belt could potentially give slightly different timing to an old stretched one but I would expect a slight improvement in performance if this was the case.
Do you think he could fitted it 'one tooth out'?

I think it could be the fuel timing has not been re set to the new belt. It's not difficult to check, you just need a timing pin set, and undo a few bolts. If it's not that, the front has to come apart again to check the timing marks. Try the pump timing first.

Cheers.