Peter Ramm wrote:

Yes - thanks - but my point is that any current which passes from my
new CU to Henley block in a failure is already passing through 16mmsq
so anything I use of greater area and less resistance in series cannot
improve the situation. What am I still missing here?

Sorry, I read your first post in a hurry and didn't take that in - just
assumed is was 25mm^2 from the meter to the service connector block.
Nevertheless, new work should comply with current regulations and that
means using 25mm^2 tails throughout unless the assessed MD is 87 A or
less and there's no diversity applied at the CU (i.e. sum of MCB ratings
in the CU is also 87 A, which is unlikely).

In all probability you might well get away with 16 mm^2, but if the main
fuse really is 100 A then the installation does not comply with BS 7671
and there is a risk of the wiring overheating under overload conditions.
Why not install 25 tails now and then get the distributor or meter
operator company round to upgrade the bit twixt meter and block at a
later date?

What final circuits and loads are connected to the CU?
--
Andy