Just out of interest, what part of the UK are you in? If you are in Scotland and the tenant has signed a SST then I would go down the route of quoting 5.12 of the SST. The landlord has a contractural right (not a legal right) to access to the property in order to lay wires, cables etc. In Scotland you might find that an unsafe installation would fall foul of the tolerable standard in the Housing (Scotland) Act and the Scottish Housing Quality Standard. Most of the time it will be a case of liaising with the housing team. If this does not resolve it then seeking eviction through court could be an option.