Engine won't even turn over

It is possible that you have a faulty alternator. You do get rare occasions where a faulty unit slips through the system and gets sold. Please read this all the way through as it give more than one way to check for where the problem could be.
How old is the battery? They are considered 'Old' once they are over 5 years old. If you know how to use a multimeter try this.
You'll need to check both the alternator and the battery. For the battery - With the engine OFF test the DC voltage of the battery. It should be somewhere between 12.6 and 13.2 volts. Anything lower than 10 volts and it could be a faulty battery. Even a fully charged battery can struggle to supply power if it is old or worn out.
For the alternator - Start the vehicle and retest the battery voltage. It should read somewhere between 13.7 and 14.7 volts. If it's noticeably lower (or higher) it is most likely the alternator.
If you don't have a multimeter you can try a basic test. Get a helper to stand in front of the vehicle. With the park brake On turn on your head lights THEN start the engine. Your helper is watching for a significant dimming in the headlights. The lights should dim as you crank the engine then brighten again. Next turn everything else on, including the stereo, A/C, courtesy light. This is to put the battery and alternator under heavy load. If the headlights dim noticeably It is possibly the alternator.
Most companies often have a guarantee or a warranty on the alternator so a replacement shouldn't cost you anything.

You'll need to check all the safety switches. Probably one in the seat, one for the mower engaged, one for clutch/parking brake engaged, possibly one for reverse gear selected, and possibly for position of the steering arms.

Hi there !

I won't say its an alternator problem because the alternator only charges the battery when its on, And it should start with the jump cables even if the battery and the alternator are bad. your Issue is not clear. Does the car start when you jump it ?

i could be that the clamps on the batterie are bad or the cable that come ut of the alternator is bad.

How old is your battery? 1st thing to check is connections at battery and starter. Make sure there is no corrosion, clean connections with fine sandpaper, make sure connections are tight. If dirty or loose connections exist, you won't be able to start or jump your car. If connections check out ok, pull the battery out and bring it to a parts store for a free check of it's condition. If it's bad, again, you may not be able to even jump start it. If battery charge is good, and condition ok, check your starter. You can also get a free check if you remove it and bring it to a parts store for testing.

sounds like originally your alternator is over charging causing dash lights to come on and flicker while driving then battery is flat(low) causing problem when starting
jump start engine, check alternator charge rate, and get battery recharged and then checked(load tested)

try disconnecting your battery ground for 30 seconds and see if your check engine light goes away. If not, i'm sorry to say but its gonna have to get scanned, it could be coincidentally so many things.

cant help a lot on this,yes it has a sleep mode mine has as well to conserve battery life,try asking your local jaguar dealer if they have come across this before and what the cure is.if they say expensive repairs then fit a battery isolator switch or a direct feed cable with a switch in cab somewhere from battery to altenator or a feed from starter motor relay to altenator

one of two things i can suggest. One being the battery is pretty dead that even a jump start won't work or the alternator may not be working.

It could be a multitude of problems, however the first thing I'd do is check that battery.

A lot of times, batteries just get wore out and even while being charged; they aren't able to help keep the system stable enough. Even with the alternator pushing out the right amperage, it may actually be causing more problems as it's not holding enough of a charge. Usually when the battery is fully charged the alternator will only power the vehicle itself while not charging the battery.

I'd try the battery first, then look into the wiring off the alternator. Also check for bad terminals, wire connections, or even corrosion on a ground contact.