I cannot find how to search, so I will ask: There is a cam chain tension device accessible under the breather cover. Can this be used to loosen the upper chain?
Yes it is a cam affair and will tighten or loosen the upper chain. The manual explained the recommended direction for adjustment. 18mm iirc.
Use the spy glass top right. Be aware that the tensioner may rotate either way depending on how the last person in there set it up.
This is from the XK120 manual.
Later on a different type of tool came along.
You have to push in the spring loaded plunger catch, then the sprocket will turn with the tool.
And make damn sure that it pops back out when you have finished.
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Anti clockwise to tighten.
Clockwise to slacken.
I see you are looking to loosen the chain. My first question has to be did you use a thick head gasket? Did your car come with a simple metal gasket? I have see “a few” people struggling with this. If full limp on the adjuster is not enough i have to wonder which head gasket you are using.
As I stated, only if thats how the last person in there did it.
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Indeed: the eccentric can, and often does, work either direction.
I know lots of eccentrics who don’t work at all. 
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