Just been hunting around. Found this on the RPI website!
Almost all used Rover V8 engines that have covered in excess of 70,000 miles will be suffering, or beginning to suffer, from worn camshaft, lifters and a stretched timing chain. This is normal with the V8 because these engines are amongst the most forgiving.
The result is a gradual decrease of engine efficiency and economy that goes quite unnoticed to start with, this is due to the fact that although you have eight cylinders, the major loss of efficiency, in only in one or two of them (typical of a worn camshaft), may not be apparent until they have perhaps failed completely, as the remaining good cylinders will mask the power loss effectively until severe enough to make replacement unavoidable. However, if left this long, many other engine components will have suffered due to this, and can lead to an engine's condition being far too bad for an inexpensive overhaul.
Why unnoticed? With the hydraulic tappets taking up the clearance as things wear away, by the time you get to hear them, they are already very worn. The initial problem will be a gradual loss of power over a few thousand miles. Also, you will notice the need for changes in carburetor and ignition settings to cope with the excessive emissions produced, seemingly from the engine as a whole but more likely the individual cylinders that are suffering the most camshaft wear (causing loss of full or correct valve lift).
This change in emissions (High CO and HC) is why most mechanics would be incorrect when they advise that your carburetion or ignition is in need of attention, when in fact the problem is more likely to be camshaft wear, along with erratic ignition and cam timing due to the timing chain stretch.
Not saying that this is definitely the cause but the bottom paragraph reads very true. Everyone saying fuelling or timing! To be fair it appears to refer to the earlier version but it still shares the basics. I've used them in the past for ignition stuff, they're very helpful and may be worth giving a ring.