Thanks Nick - I’ve also just found some 12v cut off switches for marine/boat applications that look a bit more waterproof.
https://www.ebay.co.uk/b/marine-batt.../bn_7024943528
As you say though they are larger and not very inconspicuous - I’m not really using it as a theft deterrent just to save my battery so might give one of these a go. Just off to look exactly where to fit it and will order one and the right sized cable and have a go at fitting once the bits arrive.