Thank you all for your advice! Looking at those locations you have suggested, the address on the 1871 census seems to be not too far away. 1871 census has the names indexed as MATHUES, and the address I think is 12 Burnley Place, (or Rd) Lambeth (Bunnily Place on the initial reference I had - but I think it's Burnley, after looking at the image again).
The whole family is there, including Robert. So not too far from Fulham?.?
The image shows LAMBETH, Parish of St. Michael, Sub Registration Kennington Second, (Kensington?) Ward of Stock Reville, Dist. 15.
On the question of her husband's death at sea in 1873, firstly there was a notice in a Victoria (Australia) Gazette, stating the death of John Gibbons Mathews, his full name! Died at sea on ship Northumberland.
Another Rootschatter found that one for me, and more recently I received from a different Rootschatter, a copy of his death entry in the Marine Register of Deaths, and a Passenger List Snippet. Neither of these had his middle name, but because of the initial info in the Victorian Gazette, where they did have his middle name, that was definitely the right person.
It's the death entry, in the register by the Ship's Captain, that states "Widow", so although initially I took that with a grain of salt, the entry in the Gazette was a match!
He had a sister in Victoria, so a motive for going there, and if his wife had died, probably even more of a motive! I have since looked at a copy of the Passenger List, just in case she was on the ship with him, but no sign of her on there, and likewise have found no sign of her on England 1881 census either!
So I'm going to go ahead and order the Robert William Mathews Death Cert. and maybe get some more info from that, but won't get it until Sept., as I will be in the UK until the end of August. So hopefully there will be a nice surprise for me when I get home to NZ.
Thanks again for your help, I'll post the details of the Fulham Death cert in September when I get home! Fingers crossed !
Jeanne