Cats!

I was going to suggest you might use a heron scarer only to find it is being sold on Amazon as useful against cats and dogs as well as herons.

It's a motion sensitive sprinkler to fit on your hose. Just remember to turn it off when you go out to do some gardening.

Put 'Motion Activated Pest Jet Spray Repeller' into Amazon's search box.

Thank you Thepoisonqa.. for your suggestion but I should have mentioned that I don't have a hose nor an outside tap, I would have to fix a hose ( if I had one) to the bathroom tap as I live in a flat and there is nowhere to put an outside tap, I wish it could have been that simple. I do however thank you for your answer.

found these on ebay. dont know how good they are though.
http://cgi.ebay.co.uk/Cat-Dog-Fox-Anti-Fouling-Deterrent-Scarer-Repeller-Pest_W0QQitemZ330322430986QQcmdZViewItemQQptZUK_
Home_Garden_Garden_Plants_Weed_Pest_Control_CV?hash
=item330322430986&_trksid=p3286.c0.m14&_trkparms=72%3A1686|66%3A2|65%3A12|39%3A1|
240%3A1318

http://cgi.ebay.co.uk/SCARE-CAT-Scarer-Bird-Garden-PEST-Control-Get-1-FREE_W0QQitemZ310133706874QQcmdZViewItemQQ
ptZUK_Home_Garden_Garden_Plants_Weed_Pest_Control
_CV?hash=item310133706874&_trksid=p3286.c0.m14&_trkparms=72%3A1686|66%3A2|65%3A12|39%3A1|240%3
A1318

the second one im sure you can get from betterware or kleeneze too if you have someone in your area.

you can get a plant called 'scardey cat' think thats its name (coleus canina from 'Blooming Direct') it has a scent that cats do not like apparently and has blue flowers.
I also use a water pistol!!!!

My neighbour told me to soak used teabags in Jeyes Fluid and scatter them around the garden, which I now do. I haven't had much of a problem since I must say, however I do have a dog which might help (though she loves cats!).

this question comes up quite often, Labyrs50 (not that I mind replying!) so after looking at these suggestions, you could try doing a search on past tips.
One of the best ideas is to go on the attack and buy a good water pistol to shoot at the offenders when you see them. Gets rid of some frustration too!

Thank you everyone for all your very good suggestions, I really appreciate your help, and yes Weeding thank you I will look back on previous suggestions when others have asked too. It should have occurred to me that others would have asked the same question, I imagine I am no different from anyone else who think ones problem is unique...lol I do have a spray bottle that when I am out in the garden, I first of all threaten them with it, if they stand their ground, then I start spraying. My problem is they have got wise to me, now they only come in the garden when I'm not in it and do their dirty deed, such as in the dead of night or when I'm out. Crafty little critters..lol

a spud gun does a similar job its the noise that scares them and you have the satisfaction of 'shooting them' with out doing harm.

Lol Seaburngirl, I'll look around for one.