Hi Supergran,
By all means contact me when you are ready, I'll be happy to help you, I go three generations further back than Joseph and Sarah Willis.
Have you thought about asking on the Essex Lookup Requests for some kind person to have a look in Boyd's Marriage index for William Willis marriages, you never know your luck it, could be in Boyds.
Another thought, have you, forget that thought, I was going to suggest an 1851 census check to see where William's wife was born, could have provided a clue, then I remembered the 1851 for Stebbing and some other Essex places doesn't exist, der!

If you haven't already done so a check of the 1861 census might find him and his wife still alive and kicking, William would be around 68 so there is a chance.
By the way regarding the Joseph Wills of Devon, one person had him married to a lady in Devon with all children born there, plus Sarah Thurgood, plus another Thurgood lady, all their children born in Essex. The children's births/baptism overlapped so all I can say is that if he could have achieved that, he was a better man than I, the travelling alone would have killed him

Regards