From PANS-OPS Section 4.7, Missed Approach Procedure While Circling:
If visual reference is lost while circling to land from an IAP, the missed approach specified for that procedure must be followed. It is expected that the pilot will make an initial climbing turn toward the landing runway and overhead the aerodrome where the pilot will establish the aircraft climbing on the missed approach track. (etc etc)..different patterns will be required...depending on the position at the time visual reference is lost.
From Section 4.8, Visual Manoeuvring Using Prescribed Track:
The missed approach for the normal IAP applies, but the prescribed tracks provide for manoeuvring to allow for a go-around and to achieve a safe altitude thereafter (joining the downwind leg of the prescribed track procedure or the instrument missed approach trajectory).
My copy is dated 25 Nov 04, don't know if it's changed since then. Not sure if that helps.