Yep Daltons was the recommendation I followed about 10 years ago (probably from MudRunner) with our TD42. They are wizards in the black art of tuning TD42’s (& very possibly other things too). They gave me exactly what I asked for & it has never needed touching again & from memory their prices were very reasonable. Fitting a 3” exhaust & taking our car there was the best thing we ever did to it. I’d say they would be worth talking to about your ZD & if it’s not their thing I reckon they’ll have good info on where else to go.