Don't get the head skimmed again. Landrover say you can't skim a TD5 head at all but you can get away with it once if done properly. AFAIK there is not enough "meat" on the head to skim it twice. Always replace any nylon locating dowels with steel ones to stop the head walking on the block over...

I changed all my oils and filters every 5k miles. For what they cost its cheaper than an engine rebuild or replacement.

Are you sure it's white smoke and not steam? Black smoke is unburnt fuel, blue smoke is burning engine oil, white is normally steam.

Take the top cover off and switch the ignition on. Do not start the engine or you'll have oil everywhere. Have a look around the injector pockets while the pump is running. If the O rings are leaking you will see fuel bubbling up.

Use an oil filter strap or chain wrench around the filter rather than Stilsons. Try tightening it a tad before loosening.

Easy enough to do yourself. Make sure everything is clean before reassembly. No special tools needed. Even easier with the engine on a stand.

Yes you could do that. The ECU will quickly decide the MAF signal is not present and switch to a default mapping with preset MAF values. If the power output picks up then the culprit is the MAF sending unreliable signals but as there is a varying signal present the ECU will not switch to default...

It has to be something to do with the head or injector seals as there is nowhere else the engine oil and fuel can come into contact.

Unplug the maf and see if things improve. If so there is your answer.

I'm certainly not plugging anyone! What they did with it after buying from me is up to them. MOT history is readily available to those who want to check. Caveat emptor or buyer beware as the saying goes. Although my Defender was one of the good ones so somebody got a decent motor.

Don't use thread lock.

I guess that's why Landrover did it then?

As above use some multicore caravan cable and the jobs a good un. Use the old loom to pull the new cable through the chassis or run externaly.

When I sold my 98 TD5 90 CSW earlier this year I was astonished by the interest and price it fetched. Given it was an 18 year old Landrover it was a very clean and tidy example with a few minor scratches as expected. It was bought by a 4x4 dealer who paid me £900 more than I was initially...

Maybe the oil pump bypass valve spring is broken or weak or the valve is stuck open? As said you really need to measure the actual pressure with a decent gauge before trying to second guess the cause. The ticking relay is a strange one though. Does your motor have an oil pressure protection...

From memory the ring should be a separate piece. I don't have my Defender anymore to go and check.

The main problem you need to guard against is engine runaway due to excessive oil levels. I would be checking and adjusting it two or three times on a 500km run just for peace of mind. The TD5 is designed to run on a relatively thin engine oil so using a thicker oil that becomes diluted is OK...

The exact opposite. They were initially designed for critical use in ally parts where the base material is not hard enough to take a thread subjected to high torque or repeated use. They are commonly used on aircraft and aerospace technology and well proven. I have used them on several occasions...

Prise out the centre cap. Loosen the nut but don't take it all the way off unless you are using a puller. Pull back on the wheel and give the shaft a sharp tap with a suitable implement. The wheel should come loose. If you remove the nut completely you'll likely break your nose when the wheel...

I think it is safe to say there is more chance of the OP being kicked to death by a chicken than somebody supplying an MOT certificate from this forum. JMHO :D