Take great care!
You may have worn ROCKER ARMS where they lean on the tips of the valves, which would easily give you a false reading using feeler blades between the rocker-pad end and the valve tip.
There are ways round this, like a set of new rockers, but there's an even better way.
Ain't there always if you ask?
First, valve clearances are NOT critical EXCEPT (a) you MUST have some and (b) it gets clicketty noisy if you have too much. The ideal is SOME ALWAYS, but not a lot.
Let's assume you have NOT swapped valves or rockers around. They are all where they always were and should be.
Start at the first valve. Turn the engine till the LAST valve (no.8 at the back of the engine) is on full drop. Undo the locknut on No. 1 rocker, and VERY GENTLY screw down the adjuster till the rocker tip touches the end of the valve stem. DO NOT SCREW DOWN HARD as you WILL push the valve open! Just adjust down till the rocker reaches the valve stem tip. Do NOT slip a feeler blade in to check it's touching as that will ruin the whole plan. At this stage there should be NO slack at all - zero lash as the Merkins say. All the slack and wear even in the rocker bush will be taken up. You can feel the clearance reducing to zero as you screw the adjuster down. Now the clever bit - UNDO THE ADJUSTER A LITTLE LESS THAN A QUARTER OF A TURN - that's let it UP just under a quarter of a turn. Hold it there and tighten the locknut firmly Job done. You now have between 0.008" and 0.010" clearance. Ideal.
Now turn the engine till valve 7 is full drop and adjust valve 2 the same way.
Then drop 6 and adjust 3
Then drop 5 and adjust 4
Then drop 4 and adjust 5
Then drop 3 and adjust 6
Then drop 2 and adjust 7
Then drop 1 and adjust 8
It's called "The Rule of Nines" and applies to most Landies and Minis.
I have a tool here that sets valve clearances this way.
Does anyone else have one of these?
I haven't used it since 1975 I reckon, but up till then it saw a lot of service.
Good tool once you get to "know" how to make it work well.
I might be persuaded to part with it .... sob .... my faithful old friend ...
but it should go to a good home ...
CharlesY