Looking down the steering column once the steering wheel is removed you can see the 2 indicator cancel triggers (marked with a blue dot) and the lug/cam that clips around the steering shaft marked with the green arrow.
Once the steering is centralised then the indicator cancel needs to be reset which is a simple matter of gently tapping the lug/cam clip to a central position between the 2 triggers.
I did this whilst the steering wheel was off but otherwise the same can be done by removing the steering column cover held in place by a couple of screws.
MGB tips mods and maintenance 