cav injector pump JCB 3cx

Now you need to remember to run Bio-Bor additives in the Diesel, and/or drain and flush the fuel tank at the end of the season.
You either store it with the tank full to the brim (so moisture doesn't condense and/or air gets in to oxidize the fuel) or bone dry, and run the fuel through something else to use it up.
And having a fuel filter before the first suction pump is a good thing - Because when the engine is at idle the excess fuel circulates back through the relief valve, and return line to the tank, and the primary filter will scrub out the worst of the gunk over time. If yours isn't set up like that, you might think about changing it.
You can also make a portable tank cleaner with a diesel fuel filter, small electric high-flow fuel pump, and some hose. Vacuum the gunk out of the bottom corners of the tank by manipulating the hose end, as you pump it out and into a separate tank. Then pump it back to the tractor tank and filter it all again.
My total guess answer (IE take it for what it's worth) is... There's a lot of stuff that's marked "No User-Serviceable Parts Inside" just to dodge the liability.
If you have a good idea of what you are doing and use some care, you can. That means having the right tools and parts, with proper $25 Snap-On or Lisle substitutions for the $500 "Manufacturer Special" tools.
Get hold of the service manuals from JCB and whoever made the injector pump (Lucas?) and see what they say. If this is the shutdown solenoid connected to the ignition, it should come apart without a whole lot of drama, but the book will tell you this.
And the service manual will give you the part numbers for the rebuild gasket kit and the valve internals, which you'll want to replace as you pop it open and scrub it out.
It will also point out which screws NOT to mess with - like the ones that adjust the fuel output pressures. Unless you want to go get the special pressure gauges (or build your own...) and rebalance everything when you are done.
And even if it's mute, the exploded diagram will show how it comes apart and goes back together. And where that little check-valve ball and spring that falls out is supposed to go back when you button it up - Leftover Parts are a Bad Thing...