The house is a large 4 bed detached. The circuit in question originally served a very large living room, a decent sized dining room and a very large bedroom above. A large conservatory was added at the same time that the garage was converted to 2 small rooms. The ring was extended to provide sockets in the conservatory and these 2 rooms. The break on the ring is such that 2 sockets are one on leg and the rest on the other. The highest Zs was 1.88. A 20amp breaker has a max Zs of 2.3, however once the rule of thumb is applied this max Zs comes down to 1.84. Thus the downgrade of the breaker to 16amp which has a max Zs of 2.29
The house has a newish board. It has one RCD for sockets etc with the lights on the non RCD side. Bonding is 6mm for water, 10mm for gas