After 2 alternators and 3 regulators in 18 months I decided to solve the problem by getting a modern alternator fitted to my 72 E-type.
One of the problems with the orginal BUTEC setup is that a bad alternator can fry the regulator... so replacing the regulator sometimes will not help since you'll be back to square one, minus a regulator...
While the conversion is simple on many cars, on the V12 E-type it's a little more tricky because the alternator is mounted backward, turning the wrong way. If not a problem to generate electricity, it's a problem for the cooling fins...
The shop ( Jaguar Doctor in Boca Raton FL) came up with a solution : mounting the alternator where the air pump used to be, turning in a conventional direction.
A tensioner had to be fabricated for the main belts since in the stock setup the alternator is used to tension the belts.
In addition to better reliability, modern alternators are half the size, weight,... and costs... as the original truck based BUTEC.