They just clip in. Sometimes genuine ones appear on eBay but don’t be tempted by the cheaper versions, they don’t last long and the fit is quite loose.
Find the alarm chirp sounder, under the wheel arch liner and behind the driver’s side headlight, and disconnect it. These Saunders have rechargeable batteries that degrade over time and short out causing a massive drain on the main battery.
Quite possibly due to the position of it. Pretty sure the rear subframe has to be dropped to change it, so all in all, quite a big job for something relatively small.
I agree though if you look at how much electronics they’ve stuffed in the wheel well, it’s understandable why there isn’t a spare.
I would prefer less electronics and proper safety equipment.
Check the condition/charge of the battery and if need be, give it a full overnight charge.
Check the connectors going into the CJB (central junction box) for signs of water ingress (verdigris).
First thing to do, locate the “alarm chirp sounder” and disconnect it. This is the most common area for a drain. Also check that the flap in the ignition barrel is closing when the key is removed.
Welcome to the forum Coby. By all means carry out a full service but also check all he air hoses are tight, secure and free from leaks or damage. Above all, have the codes read as anything else is just guesswork.
If you are 100% sure the valves are opening and closing then the head will have to come off. I would suspect worn valve seats or piston rings leading to the lack of compression in the two cylinders.