On the planning portal it says:

Single-storey rear extension must not extend beyond the rear wall of the original house* by more than three metres if an attached house or by four metres if a detached house.

* The term “original house” means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date). Although you may not have built an extension to the house, a previous owner may have done so.

Our house has a bay “wall” at the back (one side of the house extends further into the garden than the other half).

Is the 3 metres measured from the bay wall, or the wall-wall?

Both. 3m from the bay in line with the bay and 3m from wall-wall for the rest so you could effectively build the same shape back wall again, just 3m out

Thanks