... "L on RH bend sp Cortachy (B955)".
I'd prefer "L on RH bend B955 sp Cortachy"
The brackets might confuse me a little as this might imply its not actually the B955, but a road signposted as leeding towards the B955.
By analogy: "L on RH bend sp xxx (M1)" would imply towards the M1 while "L on RH bend M1 sp xxx" would imply joining the M1 which would of course be illegal.
As another example: "SO at X Axxx" would imply straight on at crossroads, onto the Axxx, while "SO at X (Axxx)" would imply crossing the Axxx, and "SO at X sp (Axxx)" would imply the road is signposted as leeding towards the Axxx.