One/both (3.6 TDV8) of my head gaskets have failed between combustion and the cooling channels. I have ruled out other potential causes such as EGRs and thermostat housing.

It's time to trade the car in and get another. What's the best head gasket sealer I can chuck in?

I read Steel Seal or K-Seal. Any recommendations? 2015 Range Rover Autobiography 4.4 SDV8

None of the aforementioned. None of them work. The combustion pressures are way too high for anything like that to have a hope of working.

check out this post

https://www.fullfatrr.com/forum/topic54543.html 2011 TDV8 Vogue SE

Might as well just pour a bag of sugar into the cooling system with that mate instead of k seal etc

So how do I get rid of the car then?

Just trade it in? 2015 Range Rover Autobiography 4.4 SDV8

Depends on who you plan on selling it too. If you go down the wbac route then you'll prob be OK but other dealers may be able too chase you for a faulty sales of goods route

Ok, I won't bother with any liquids - I'll try selling privately with a head gasket problem and if not, I'll be sending it to auction and cutting my losses. 2015 Range Rover Autobiography 4.4 SDV8

Probably the best option. However, before you do, has it been confirmed as a head gasket? Might be worth shelling out £100 or so for a decent garage to perform a cylinder leakage test just to be sure. Please note there is a big difference between a compression test and a leakage test. A compression test is fine for a burnt valve or suchlike, but a leakage test is far better at detecting a head gasket leak, especially if it is only minor.

Compression test; Imagine trying to blow a crisp packet up with the other end torn open, you'll never do it.
Leakage test; Now imagine blowing a crisp packet up that only had a pin hole in it. You'll blow it up but it will eventually go down.

The reason I'm labouring the point is that if you have a leakage test and it fails, then at least you know for sure one of the heads will have to come off. However if it passes, you may find whatever coolant leak you have, it's not a head gasket and could end up being something relatively straightforward.

If you do decide to get it checked, make sure the garage knows the difference between the two tests, as a surprising amount don't.

That seems sensible Sam, otherwise Gary you could be throwing away a lot of money.

How much is a head gasket repair vs. Uplift in the value of the vehicle at point of sale.

The quote was £5k for a head gasket repair.

The car has been no end of problems, from a dodgy dealer. It will be going. I will not be spending any more money on it.

Back to my question please. 2015 Range Rover Autobiography 4.4 SDV8

Whack 2 bottles of steel seal in it, see if it improves, trade it in...