I agree with others who have carried out their own adjustments successfully that it is a reasonably straightforward procedure if you can find a level piece of ground and apply a bit of common sense.
If you've a matching pair of tyres on the front you can measure the gap between tyrewall and shock absorber tube.
This should be as near as possible the same each side.
If the tyres are almost touching the shock absorbers, you've almost certainly got too much negative camber.
Getting equal settings between sides and satisfactory feel through the steering can be just as effective as chasing optimum numbers.
Vauxhall didn't pay too much attention at the factory, so it shouldn't be difficult to make an improvement. Even a very small adjustment will make a noticeable difference to the feel of the car.
If your not confident, spending a few quid on a proper geometry check will be money well spent, especially when you consider the cost of buggering up a decent pair of tyres will be upwards of £200, however if your inside edges are already worn you don't really have a lot to lose by having a go yourself.
Bear in mind that any adjustment to camber will require toe in to be reset, too.