MtB

Another approach would be to fit some tineeee leeetle security cameras like my Tesla is festooned with. It records for about a minute whenever it sees something move nearby. Great for finding out who scratched the paint in the car park while shopping in Waitrose, and possibly similarly should someone dip the petrol tank. If it had one. Granted a load of cameras would not stop your fule getting nicked, but might capture a pic of who and how they dunnit. And ideally of their bote, assuming that's what they turned up in.

I know, but thanks anyway... But to be fair, it's Simon Wain who should get the credit as he fitted it when renovating my fuel tanks

This is exactly what I have in one of mine. A 1.5" BSP ballofix valve, must have cost a fortune!

In that case, have another one on me!

Here's a nice one for you to admire then... Hang on, sorry, wrong sort of great...

The BCN seems unlikely!

^^^Good advice^^^ I was also thinking the OP could use a line to pull the boat into and out of locks. This removes any need ever to get on the roof when locking. Also means they are already on the bank which recovers some of the time 'wasted' by using a line to move the boat instead of the engine. I've encountered a number of single-handers who use this technique for lock transiting and I quite like it. Bridges and walkways over the navigation can be a confounded nuisance though. The OP might do well to hang around a lock for an afternoon and watch how each of the single-handers coming along does it.

@KezzerN, how long is your boat and are you concerned about taking it through narrow locks (7ft wide)? Or wide locks 14ft wide? If narrow AND your boat is 45ft or more you may well need to get on the roof when going uphill but there are still ways around this. With wide locks you can probably sidle along the gunwale a few feet to reach the ladder, even with the type of handrail you have.

Interestingly I have only just discovered there are two versions of this type of step. 1) Where the step bit folds down exactly 90 degrees from the vertical fixing face. 2) Where the step bit folds down about 95 or 100 degrees from the fixing face, for fixing to a cabin side with a degree of tumblehome. I have both versions in various locations on both of my boats.

And your Waterways key? Never out of the cabin or out of your pocket? Really?!!

Ah yes. E.g."What is the most dangerous nut on a boat?"