Today, I have added an on/off button to the battery pack Ed made for me. I have used a car/lorry 12V on/off button from Halfords, 25A cabling and connectors. This button goes between the 75AH leisure battery and inverter and is meant to avoid the situation where I accidently leave the inverter turned on draining the battery as happened to me recently. It took quite some time to get this working although for some reason I have not managed to get the 12V light to come on on the switch when it is turned on. More accurately, I did get that working too when the switch was not installed on the side of the case but it has not worked since I drilled the hole and screwed it on to the case!
The location of the switch below the plug sockets is to avoid the switch foulibg the handle of the case.