Speedo drive?

[stigeye]

Common fault is usually the abs speed sensor, you will probably need a new hub which aren't too expensive if you source it yourself. It could be just the speed sensor though, either way you would probably need the codes read to determine which wheel is playing up.

Front... http://workshop-manuals.com/vauxhall/ast...placement/
Rear... http://workshop-manuals.com/vauxhall/ast...isc_brake/