We're currently running on the stock aerial for the Fat Shark transmitter and FlySight receiver.

What is the best aerial to upgrade to? Strictly 500m (distance) / 400ft (height) required, but need to keep the best possible signal. The stock ones can break up even when relatively close.

If you're using the stock linear polarised antenna (the long black one) try a circular polarised antenna like this:

SpiroNet|SpiroNet-SpiroNet,SpiroNet-FAT SHARK RC Vision Systems (http://www.fatshark.com/product/1728.html)

I don't know what range you'll get but in my experience, you'll see a big improvement.

Using a more powerful transmitter will also improve the range. I use a 650mW transmitter and the stock FatShark receiver.. and I still get signal breakups at reasonably close range. Using a digital system will solve this but they are expensive.