Cat wee and cat spray are two different things, so don't rule out a common moggy with a weak bladder just yet. Sorry.. :(
Yes, chucking the whole thing is probably best. I had a little poo on my mat a few years back. In the bin it went, and it's never happened again. Thank god.
I'd be very interested to know whether that water-spraying thing works, please.
I've got one of the Fox Watch thingies and it seems to have no effect, by the way. Mind you there's a sett just the other side of my back fence.