2001 Vauxhall Vectra cold starting

2.2 diesel, very hard to start when it's cold outside, and the cars been standing out overnight.
Does eventually start, and once started will start easily every time thereafter.
Battery check was all OK on the meter
New glow plugs fitted and working.
Injector seals were recently replaced.
Full service carried out in October.
This problem has baffled all the local Vauxhall dearlers and other car repairers mechanics for the last 4 years.
Please can you help or suggest a remedy.
Thank you..........Mr. T.M.Collins