Repellent

Does anyone know of a good repellent
that I can use to stop one of my dogs
peeing indoors at night?.
I do take them for a walk before bedtime,
but this has'nt worked so I am going to
have to resort to a repellent of some kind
to make the area unpleasant and to mask
the smell of urine,I do put pads and paper
down,but both my dogs are adults,and it
is getting a bit wearing and annoying.
I have no idea why this is happening as they
were both house trained when puppies,the chi
asks to be let out,but the bichon x does not,
and she is 4!,the thing is I don't know which one
is responsible, this has been going on since July.
Would be grateful for any ideas.