The near side front indicators had been playing up and the wiring to the main indicator had been connected to the same side of the bullet connector as the main wiring loom (thereby negating the sole purpose of the bullet connector) so it needed re-wiring anyway. Going over things with a multimeter showed that the wiring was OK but the contacts on the bulbs were oxidised. A quick clean and everything was working reliably again.
Whilst I had all the electrical tools out I replaced the off side bullet connector as it appeared to be original and wasn’t going to be an effective connection for much longer.
The off side main front indicator glass (well, plastic) was cloudy so I removed it and cleaned it out. It seems that water had pooled inside it at some point and the lower mounting screw had rusted, the clouding was the residue of rust infused water.