Nice car...and RHD

...when did that model Bentley change to overhead valve engine.
That model never did have the V8 over-head valve engine.
After that came the R-Type, and the "long boot" Silver Dawn RR, still with the in-line 6, that was around 54 to 55/57 (I'm too lazy to look it up)
Even the first generation of Clouds and the Bentley S1's still had the in-line six and it was over-bored to 4.9 litres.
It had an improved high compression head of 8:1 (if you could call that high), better breathing on the cam to let the exhaust out, and a bunch of other improvements over the first 4.25 litre.
Then in 59, the Cloud finally got the long developed 6.25 litre aluminum V8. (it was a RR engine, not a modified GM engine)
It was a good engine if you changed the coolant, examples that sat with old coolant in it corroded the aluminum and put pressure on the steel wet liners, and in extreme cases, the pistons were galled in the bores from that pressure.
Removing the liners by this time was nearly impossible to be made right, there were large O-rings at the bottom that made a seal that had to be replaced.
At this point the block would have been so corroded, that it pretty much would have been junk.
Even after heating the block and having the special tool to get the liners out, they would very often explode under pressure and/or crack the block as well.
They would not tolerate over heating when things got really silted up either, the block and heads would warp together, and in extreme cases, the long studs would be pulled from their threads from the heads expanding.
All this made worse when they added a turbo where RR did not improve the head gaskets, most blew the gaskets at around 50k miles.
Gaskets are now available that will take the pressure from the turbo, but the engine is a nightmare to work on because of the studs used to hold the heads on, just like my engine.
Getting the head(s) off can take weeks of soaking in oil due to the electrolytic action between the aluminum and the steel studs.
On my engine the studs went right into the water jacket where coolant would get in there and weld the head right on.
Lots of fun...