Hi John ,
     I doubt that would work.
 Have a chat with your neighbours about your concerns. They might be
keeping him/her inside  or you could encourage them  to keep a litter
tray for their cat or dig over a toilet patch of earth in their
garden. Cats like fine soil hence they use  flower beds .
 If you have flowers beds you could put down Get Off My Garden , safe
green crystals from Pets At Home or orange peel, or flint or pebbles
from the garden centre. You could put up netting along the top of your
fence . Bear  in mind that their are many foxes in London and they poo
on the lawn and sometimes tom cats do this . It's unfair that you have
to do this but why not wait and see what happens, if the cat does
start to use your garden then maybe the neighbour will pay for any
costs.

Alison

Use a Supersoaker and hide when firing so the cat does not associate you
with getting wet. Rather crapping in your garden. Works very quickly