I think a good start would be taking it to a Vauxhall main dealer (if you bought it at such a place, then don't go to the one you bought it from!!!). Ask them if they can help you out, perhaps they can check the engine software to give some help.
The digital odometers are pretty much un-fiddlable I believe. However, if the car drives in a lumpy fashion, it might have been thrashed in the past. Pool cars, courtesy cars, hire cars are often thrashed by people who either a) don't care about the car and/or b) aren't paying for the petrol they are wasting.
So, it is quite possible that the mileage is completely genuine, just the car has been badly driven.
There are various things you can look at to get an idea of genuine mileage:
1) if seat covers are worn then it is a high-mileage car.
2) how much tread do you have on the tyres?
3) are the tyres all the same brand and size (i.e. are they the tyres that came WITH the car)
4) Look at the documents and determine who the previous owners were: was a company the owner or was it a private owner?
5) There is an option with tax discs to record the mileage of the car. This is voluntary, but if your car claims 5000 miles and the DVLA has a record of 20,000 miles at last tax then you know there is something dodgy.
Good luck!