Planning is required for any toilet facility.
If it is a commercial activity it requires permission. Assuming the gun club pay a yearly subscription then it may be considered commercial.
Perhaps your first ports of call might be to the evironmental section and planning enforcement section of your LA. The environmental section deal with noise pollution as well as environmental pollution. The planning enforcement should be able to let you know if planning is required… but as i say… toilets definitely require permission.
If they can prove they have been in operation for more than 7 years they have de facto PP and any enforcement proceeings by the Planning authority would be statute barred. Maybe the toilet or the noise is the way to go if you are looking for a regularisation of the use of the site.