I have a Clipsal consumer unit and the Wylex MCBs in B&Q say they can
only be used in their NH range of Wylex boxes. However, on inspection,
they look the right size and are type B as per my existing ones. They
are also the same size.

Most DIN rail MCBs will physically fit and work in most DIN rail consumer
units. However, most manufacturers say that their certification is only
valid if the manufacturers match.

Christian.