Peter,  and others,

The problem with the LRP was that the different brands tend to use different additives to get the end results.

Various people have discovered that these additives aren't really compatable from brand to brand.  So unless you stick to one brand of LRP you have a possibility of corrosion and engine damage.
Vic had serious problems with his Commodore that would not run well on LRP and then there was the other "additives" some Sydney servos were adding in as well.
The low compression versions of Hillman engines run well on straight ULP, I put Flashlube or some other Lead replacer additive in with it just to be sure.  In the old days many drivers added an upper cylinder lubricant to the leaded stuff anyway.

I am going to change over to premium fuel (ULP) for my Californian because it does burn more cleanly but I will still use the additive.

Information gleaned from a variety of internet sites bears out the view that higher compression engines tend to be more likely to suffer damage and usually only when pushed at higher RPMs under load as in towing or freeway driving.  The dreaded valve seat recession.  Hillmans will announce this sort of problem because the valve clearances will close up and cause a loss of compression.  Cars with self adjusting hydraulic tappets will hide the effects of VSR for some time.
Two club pages that covered this subject in depth were the Rolls Royce and Bentley site and the Morris site.

Now the venerable BMC A and B series engines have heads made out of inferior materials and need hardened valve seats and better valve guides to handle competition use anyway.  Our Rootes products were better engineered (and more expensive when new also :–) and generally don't seem to be bothered as much.

What will destroy your engine when using poor fuels is either detonation (pinging, pinking knocking pre-ignition) or lean mixture. Like all pieces of "clever" advice this does require some common sense between 30 and say 50 mph using third gear will make improvements continuing this out to 70mph will create a whole new set of problems.  Your Hillman will do it, but it will work just as well in top gear once past 45mph.

Leaning out is a different ball game.  Using fuel that is mixed with non burning substances of lower calorific value means that the burning fuel has alll gone but there is still lots of unburned oxygen left.  This starts to oxidise (burn) the hotest parts of the combustion chamber.  This is usually the edges of the exhaust valves which quickly lose the seal, drop compression and start running rough.
Early Zephyrs with the choked up Ford "hockey stick" exhaust manifold/pipe always had lots of back pressure and unburned mixture left in the combustion chamber.  When the young hot foot removed this power sapping device and replaced it with an efficient extractor system cylinder scavenging was vastly improved.  The modified Zephyr went like the proverbial rocket, BUT, without the factory "designed" back pressure the mixture became very lean.  This burned out exhaust valves.  Old timers will tell yo yhe extractor burns valves, but in fact if the mixture is corrected they work fine.

So what do we do?
First set your timing to suit the fuel you are using.  Don't blindly use the "numbers" of advance.  Listen to it run under load and if it is knocking retard it slightly.  Use the gears :–)
Make sure that your mixture is right.  Either use the tuning tips in the Rootes books or get an exhaust gas analyser.  Silicon Chip had a design for a mixture readout that used a O2 sensor to monitor the exhaust gases that was pretty simple.  Most tune up shops can check the mixture for you.  Make sure you don't have air leaks in the intake manifold.
Rich does less damage, just consumes a little more fuel.
In the old days I always ran engines on the rich side for this reason.
Most of my early Hillmans tended to run either closed throttle or wide open so I never worried too much about fuel consumption :–)

I'll leave it there for the other experts to comment :–)

BTW the Californian will be at Warwick but still 1390 "powered".  I decided that a 1200km shake down trip for a new and unproven engine would be a bit too stressful. So I will just give it a tune up and tighten up the bits that seem to be dropping more than the "normal" amount of oil on the floor.

Keith