It's important to get the rug on early enough in the season to prevent the pony from being bitten at all. I know the Rambo sweet itch hoodie rugs are good and, at £80-£90, are less expensive than some.
I use 'Click' which is for fly control on sheep. It gives protection for up to sixteen weeks when used on the manufacturer's recommendation (for sheep). It's probably not licenced for horses, but I haven't had a bad reaction yet. It leaves a pink or blue stain for a week or so, but that fades.