Daf Lf45 2010 start issues

Evening all, first time poster and self admittedly not massively technically minded!
My daf was reluctant to start a couple of weeks ago. The lights etc were coming on and it was trying to start but wouldn’t catch. Eventually I just held the key in the start position and slowly it started. I went about my day and no further issues with it starting or running.
Went to start it today and same thing but this time it just wouldn’t start. It’s been sat a fortnight (it’s a horsebox). If it was the battery would the battery warning light not come on? .I always isolate the battery when it’s parked up.
The only warning light that came on this time (other than brakes of course) was the emission light which was yellow not red. It has plenty of adblue but I know it’s a bit of a known issue. That wasn’t on last time I had a problem.
Any advice gladly received before I get someone out

The red battery symbol should (1) come on as soon as you turn the key to the first position. It should (2) go out when the engine starts (and you have blipped the throttle maybe). If it doesn’t do both then the alternator is not charging. Although the isolator has been activated the tachograph still draws current and will flatten the battery if the vehicle is left parked for days on end. Horseboxes, which are typically left parked and isolated for weeks and months on end, can then exhaust the battery in the tachograph, this will kill the tachograph resulting in a very expensive replacement invoice. Your friends will tell you that you don’t need to use the tacho which is partially true, but your vehicle will not have a working speedometer if the tacho is dead and there will probably be gear selection issues if it has an automated gearbox.

Get someone to test you batteries and confirm the vehicle is charging. You can buy special battery chargers which can safely be left permanently charging the batteries at a very low top up current.

partially To carry other people’s horses you must use the tachograph and become subject to other regulations as well.