Hi Heather,
I can't give you any figures, but I know couples did pass themthelves off as being married...when they clearly were not!
My own 3xGt Grandfather was living and working in Salford (Greengate district) as early as 1870. His first wife died and then he was soon living with his new partner (described as his wife on the next two census returns), but I know for a fact that they did not get married until some 20 years later after they had moved to Middlesbrough, which is where I found their Marriage Certificate! Why did they get married so late on? Legal/Probate reasons?
Regards,
Mark.