Hi Everyone. The heat output has always been mediocre. It’s just not warm enough. The engine temperature is fine. I do think it’s the heater valve. How much of a pain is it to get in their? It seems to be in the most awkward spot.
Do you mean this heater valve?-
https://www.jaguarclassicparts.com/uk/jaguar-xj6-xj12-parts/engine-cooling-system/radiator-and-hoses/water-valve-3-2-4-0-litre
If you suspect that it is not working (i.e it is permanently closed, preventing hot coolant from entering the heater matrix in the HVAC unit under the dash), feel the feed and return hoses to/from the firewall when the engine is up to normal operating temperature. The temperature should be similar in both hoses - i.e hot - indicating that coolant is flowing through the firewall to/from the heater matrix.
Remember that on the '90 - '92 models Jaguar deleted that heater valve so in extremis you could remove a defective valve and with a little plumbing artistry feed the coolant direct to the heater matrix.
Not so good in summer though - degrades the already questionable AirCon efficiency! 
I’ve determined that the heater valve works fine, even though it leaks a little. It is staying open by default and closing on vacuum when the temperature control is moved to the colder settings. Hot water is flowing through it and to the heater core. As soon as I have the motivation, I’ll check out the blend flaps. Not only is the air just warm, there’s not much of it coming out of the front foot wells. There seems to be some coming out of the defrosts vents when in the heat position, very little from the front lower vents, and a decent amount from the rear seat vents on the side of the console, or on the back of the console, depending on the position of the rear slider control.
Hi Darren …
Getting heat into your cabin depends on a couple of things …
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Is the hot engine water getting to your heater core? As mentioned the water valve defaults (vacuum removed) to the “open” position. I guess the Jaguar rationale would be if the system failed and worse comes to worse you won’t freeze to death
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Is the hot water actually running through the core? in another words is there a blockage somewhere inside the tubing.
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Is the air actually passing around the heater core fins? Your car is two and a half decades old and having no air intake filter a LOT of crude has past through those fins and may have built up and is now partially or completely blocking the air flow.
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Are your two fan assemblies working? (easily checked by pulling one then the other fuse)
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Are the 2 blend flaps working properly? There is a step by step test to check this.
As usual it’s probably not one thing but a little of this and a little of that !