Hi Thornbird,
Since wlfndl has resurrected this thread after it being dormant for a few years, have you managed to find out anything useful about your grandmother in the 1921 Census? For instance, if she was co-habiting with anyone, or whether there was a William Woolfenden living in the same neighbourhood etc? One assumes that since they had more than one child together, the relationship was more than just a fling, even if he was still married to someone else.