Nope, not LH. 8mm w/ 13mm head
Sheared off? - drill and extract.
Rounded off head?
The new filter will hang for a little while until you get the bolt out. Zip-tie it.
Ed7

G' day folks,
I'd been noticing my temp gauge running lower and lower over the past couple of weeks.
Now I've a cold running engine - gauge barely gets off of the bottom. Coolant level is good.
The cooling fan is on continuously - from start-up.
The engine does not go through the warm-up cycle, but ...

There are few diesels in the US, and fewer on this board.
Quite possibly, you could have leaky injectors. You mention the car sat for a while. A fuel additive expressly made for diesels may be helpful.
A diesel shop may have an exchange program for injectors that is not as expensive as the ...

The pinion gear is still engaged with the flywheel when it should retract.
This starter is in need of replacement. The bushings are worn and is beyond repair.
Ed

Nice close-ups.
Don't over service the filter.
The big crud doesn't go through, but helps stop the small stuff.
Ed

A thin coat of hi temp grease or antiseize on the mating surfaces on reassembly is a good idea.
Ed

The wire goes to the fuel door lock solenoid.
Ed

Excellent question.
That is your manual fuel door release.
Ed

The great thing about it is that you'll never know when it's clogged.
The majority of transmission debris is nonmetallic from the friction discs.
Ed

A broken wire is common under the car. The solenoid doesn't make a lot on noise, it's barely noticable.
If you're getting voltage on the solenoid wire under the car, the solenoid/valve is shot.
Ed