There can be many reasons for misfiring. You can try the following solution one by one,
1. Change Spark plug (champion plugs) and magnecor wires
2. Clean MAF and stepper motor
3. Do the whole decarbon and engine flush treatments
4. Clean or Replace cylinder heads
Have you checked the timing? The timing is the reason though, check the belt, the gears and it is an easy fix. Most garages are able to fix this at a reasonable rate.
Typically misfire are caused by loose or failing components. Go over all the components and check to see they are functioning. It also helps if you get a good tune up on the engine.
You could have some build up from cheap gas or one of the cylinders in the engine could be off. Try changing the gas to high quality gas. If that doesn't work have a mechanic check it.
Your gas tube on your Land Rover may have a possible hole or tear in it. If this is the case you would need to purchase one and have you or a mechanic put it on.
Engine misfire is due to a clogged fuel pump and an unclean carburetor and spark plugs, clean up these components and you will not face this problem, if the problem still continues check with the service station.
