Tim Watts wrote:
Is there a way?

I'm getting a tad tired of the number of piles of mouldy cat[1] poo
appearing of late.

[1] Almost certain it's a cat - there are sometimes vague attempts to
scratch the ground and bury it, but often not successful. Volume and
size is too small for a fox.

There are 3-4 cats that appear regularly.

I know it's not going to be easy - but I was wondering if there is a
chemical I can spray around the permimeter that is either deeply
unpleasant to cats or makes them regard it as marked territory?

I don't mind the odd cat, but they are becoming a right pest lately.

I've had great success with unprocessed chicken manure spread over the earth
and initially watered - it even stops my cats from crapping where it's used
for long periods of time. The upshot is that they then use the neighbours
gardens instead!