Advice on gate lock needed.

We had a visit during last night from someone uninvited who drove around the site and on to our lawn, nothing taken and no damamge apart from deep wheel marks. Although we have a caravan in they heard nothing neither did we, and I am normally a light sleeper or thought I was.

 Now we are looking for a decent lock for the gate. Are there any suggestions? It has to be a combination lock so visitors can easily use it. Any advice gratefully accepted!