Backyard Stoush

Since we moved in we have noticed a few cats hanging around, rather big cats to be precise, but in the absence of any noticable territorial marking smells we haven't bothered too much about it.  This week we decided to pay particular attention to the new neighbours and have counted 4 of them. 
On sighting we usually run outside yelling and hissing to scatter and deter them but last weekend I suggested that we should see if they seemed friendly at all - big mistake.
The very next day after said experiment I caught one of them abluting in the back corner of our yard so I immediately returned to charging at them growling and hissing.
On waking this morning, 2 of them were hunkered down in the not so recently mowed back "lawn",  maybe fantasising that they were African lions on the plains, strange choice though given it hadn't stopped raining overnight and was < 10 degrees outside.  We observed them for a moment but then I couldn't resist charging at them again to see what happened.
The pair of them ran straight for, and up, the back fence, not a bad feat for what I originally thought were some over loved pets.
We watched eating our breakfast quite leisurely as they faced each other off for @ 15 minutes, eventually Mr Grey started to back away.
I got bored with the stale mate so charged at them again, they both leaped over the fence and disappeared.
So, I think I either need to get a dog or a nerf gun...  figure the latter might be quite a bit less maintenance and expense, any suggestions appreciated.

 Round 1

 Round 2

 Round 3