High Wire, you are correct although its a bit of a long story, I'll try not to bore you all.
Caterpillar wanted to get their hands on Rolls Royce military diesel multi-fuel technology and also to buy JCB excavators.
Perkins bought Rolls Royce so in turn Caterpillar then bought Perkins and some engines like the Caterpillar C12 are actually Rolls Royce designed engines.
As JCB used Perkins engines Caterpillars boardroom avarice at work said 'if you won't sell us your company we wont sell you engines for your excavators'.
So JCB promptly set up an engine factory and build their own engines for their excavators which have also become a firm favourite with fishermen because of their reliability and superior hydraulic systems for the trawl nets.
JCB also hold a land speed record for their diesel engines.
I also mentioned
www.parts4engines.com this company supplies Perkins with OE parts for engine assembly.
Perkins parts are generally cheaper than Caterpillar and as boatpoker says agricultural dealers are definitely far cheaper than marine.
Raw water cooling parts/heat exchangers etc can be obtained from
www.ejBowman.co.uk who are Perkins (and many other) OE suppliers.
With Great Britain's exit from Europe's restrictive practices if its made in Great Britain you can now get whatever you want, wherever you want.