Originally, I thought ‘What a nasty forum – if this is how you welcome a newbie, I’m out of here’. Then I realised it was Friday night and a few beers might have been consumed after a tense working week. That may have been the reason but was not an excuse.
Thanks to those who made on-topic, constructive responses.
The outbuilding is to be a pottery studio and will have lights, a socket ring and heater spur (but no kiln, thank God) and is 50m from the origin CU, therefore the calc is just into 10mm2 to avoid excessive volt drop.
This new submain would share half of a split board (large house) and outdoor stuff is always at risk of tripping an RCD. A mate has advised me to give the shed and outdoor sockets a dedicated RCD/RCDs and also that I would have a distribution circuit doubling as a final circuit and it should be one or the other (as per Wirepuller).
I spoke to the customer yesterday to urge her to let me feed one outdoor socket from the shed mini CU (local RCD) and the one nearer the house (25m run) from an RCBO in a new mini board within the house. It’s a lot more cost and if she doesn’t buy it then I will have to cut the 10mm2 and put sockets on it as per plan ‘A’.