The latest version of the Leak Sentinel arrived yesterday. For 200 euro it seems a good deal.
Apart, it looks like this:
The cap is removed, the red valve depressed to switch on a micro-switch on the circuit board, a hand vacuum pump attached and pumped until the flashing red LED turns to flashing green. This takes about 25 pumps with my Nauticam D800 housing. The cap is then replaced to keep water out.
If the light remains green, then there's no leak. After the dive, remove the cap, and push the red rubber valve to one side to allow air to enter. Press again to switch the circuit off.
To replace the battery, the three Allen-head screws must be removed, not a difficult job.
It is installed in the 16 x 1 threaded hole in the housing. I used some thread-lock to prevent it being unscrewed accidently.
I will take it diving in a couple of days.