John Rumm wrote:

Hager do a MCB sized HRC fuse carrier, which can solve that problem in
many cases....

Oops, quite right, I forgot that option.

although having just typed that, I remembered that
Roger's CU ain't a DIN rail jobbie anyway, so this is of little
practical use to him! ;-)

Doh! - his picture tells all, and I guess the new circuit is going in
the empty fuseway on the right (currently 15 A). In this case the
obvious thing to do is to use a 30 A BS 1361 fuse and not another
plug-in MCB.

--
Andy