Woland wrote:

I have veen following anxiously the issue of electrical regulations,
XVII ed. (part P?)

The 17th edition and Part P are quire separate things. The latter
doesn't apply in Scotland.

Now the problem: in 2000 [...] (there were no split units at the time).

Oh yes there were.

A 49Amp MAX armoured cable (should be 2.5mm^2, not certain but I'm
certain it's 49 Amp MAX) which is some 40 metres long goes from the
RCD-protected CU straight into the summer house to provide power.

With that length of run the effective current rating of your submain
will be limited by voltage drop and not by the current rating of the cable.

You might find
http://wiki.diyfaq.org.uk/index.php?...ricity_outside
useful.

--
Andy