Luckily I did have a spare but I would like to repair the old one for a back up and discovered this Ford one that looks almost identical to the Bristol one whilst searching Google
Early Ford Store of Ca - Early_Ford_Parts New and Used Original Parts for 1928-1959 Cars & Trucks
Hopefully the four bolt holes will be the same or near enough to make it fit ! They also sell various clutch and spring repair parts, so it may be possible to use one of them to repair the original -- I will know when I have it in pieces :-)
Be careful to secure / put something under the glass when un bolting the regulator as it drops like a guillotine if you are not careful !!
Hope this helps someone