Restaurants are free to charge what they like for corkage but they tend to base their charges upon the amount of profit that they'll lose by permitting you to provide your own booze.
For example, a local (i.e. not especially posh) restaurant might buy their wines for around �4 per bottle and sell them �10.50 per bottle. So they expect to make �6.50 profit on each bottle of wine consumed on their premises. They might decide to charge this amount as corkage. Alternatively, they might only charge �5 per bottle, out of goodwill, or �10 per bottle, to discourage customers from bringing their own wine).
A more upmarket restaurant (i.e. the type of place that hosts posh wedding receptions) might buy their wines for �10 per bottle and sell them for �30 per bottle. So a sensible starting point for calculating a corkage charge would be �20 per bottle.
�3 per bottle corkage on wine is exceptionally cheap.
Chris