Stuart Hoskins Feb 7, 2015:

balancing responsible party (BRP) This term has become widespread since the question was asked.

Dylan Edwards (asker) Mar 10, 2011:

Thank you, that looks useful.
I was inclining towards "entity".

Stuart Hoskins Mar 9, 2011:

Elexon I generally look up elexon (elexon.co.uk) for this sort of stuff – they have a glossary and documents explaining the electricity market in layman’s terms. I think the closest I have come to “subjekt zuctovani” in English is the unwieldy (and for your purposes unusable) “party to the Balancing and Settlement Code”. Personally I use “settlement entity”.