I know it's a bit of a treck, but I always take my Vauxhall to Andy Stockton in Widnes. He is an independent Vauxhall specialist and usually uses the OPCOM to confirm his diagnosis, rather than make it. He knows the cars inside and out, gives excellent service and is good value.
As I said, I know it's a bit far away, but he's never failed to fix my 'problem' zafira, so he's my first port of call and to be honest, the only reason I'd buy another zaf.
A common problem with diesels (certainly astras) is leakoff pipes. They make starting a real problem and some garages replace everything first before checking them. If you park overnight with the car pointing down a hill, does it start any easier?