If you have the alko 3004, then I beleive you are tied to the alko hitch lock. It's not cheap at around £70 but does stop your hitch getting knicked as it covers the fittings and can be used while hitched to the car too which usually means your insurance will cover you if you stop en route. Some companies insist on a wheel clamp being fitted too which is one of the reasons a changed to Caravan Guard as they do not require a wheel clamp while hitched up. That and the fact they were £5 cheaper.