You should keep a wary eye on this, as tappet clearances opening up indicate WEAR somewhere in the valve-gear. Of course, they might just have been badly set long ago, and you just noticed it. People do make mistakes setting clearances.
If it is due to wear and tear, it could be .... (some list!)
worn lobes on cam - but it would be VERY noisy and running a bit off
wear in roller-cam-followers.
Wear on ends of pushrods or bent pushrods
wear in rocker bushes and / or rocker shaft
wear on tips of rockers and / or valve stems
However, LandRover were quite cunning when they built these engines, and you will see that the rockers are NOT equal lengths either side of the shaft.
The pushrod side is a LOT shorter than the valve side, so in effect the lift imparted by the cam lobe is increased at the valve stem. This means that 0.010" ten thou (quarter of a millipede for heathens) clearance at the tip of the valve is a lot less at the camshaft end, and so the system can stand a bit of excess clearance for a fair time, but in principle excess valve clearance isn't good.
Set it spot on at 0.010" each valve by the "Rule of Nines" in these engines, and in six months time if you can't leave it alone you could lift the lid and check it again.
CharlesY