Stephen Dawson wrote:

So basically the answer is yes the C50 is obselete and the breakers are
expensive to get anyway.

The breakers are still listed in the current manufacturer's catalogue
and are available from wholesalers and several web sites. They're now
made to BS EN 60898 (Type C, M4.5) not the old BS 3871. I wouldn't call
that obsolete. Nor do I see any particular need to replace them in an
older installation which was designed and wired before BS 7671 even
existed (during 14th or 15th Edition eras, most probably) - provided
there's no evidence of damage or deterioration of course.

--
Andy