Astra Diesel Tuning

Has anyone had their Astra-G diesel tuned and if so, who is recommended
(ideally in the Midlands)?

I have the 2003 2.0 DTi SRi (100Bhp) model if this helps...

 

Surely if its a 2003 model its still under dealer warranty - in which case
tuning is part of the service schedule, and you need to have it
serviced/book stamped to keep up the warranty.

Even if a grey import (car supermarket) with just the standard 12 months
warranty on it, the same applies if you purchased an extended warranty

and even if not, if there's a problem perhaps it's still just a service
matter ?