I think what we are trying to say here, is that from a rabbit welfare perspective, the tethering to a stone, regardless of whatever weight that stone is, of an animal who is in essence a pray animal, is not really in the best interests of the welfare of your rabbit. As guardian of your rabbit, you assume full responsibility to the protection, welfare and well being of your rabbit at all times. Your rabbit relies solely on you for it's protection and wellbeing. If your rabbit has a hutch, then this is a place of sanctuary for your rabbit. It needs that protection. It needs to feel safe from harm, from predation and threat. The hutch will allow your rabbit to feel secure and likewise it will provide an environment that protects your rabbit from elements such as wind, rain and cold. And if you don't feel the hutch can provide that, then you should consider adapting a place indoors for your rabbit to have a sanctuary in.
Your rabbit is extremely vulnerable on it's own, outside tethered to a stone. It cannot run, it cannot hide. In all seriousness, your rabbit will likley suffer some episode of shock and/or possible heart attack in such a situation.
If you're garden is prone to some elements of vandalism from rowdy drunks, I would perhaps consider addressing this with the pub landlord and/or the local community support officer. In any event, the welfare of your rabbit has to be paramount to you and I am expressing a degree of concern that you feel that it is best practice to tether a pet rabbit at night without any protection.