Your quite welcome Simon.
If your looking for the cheapest place to get some, your best buying it direct from carprouk rather than one of the myriad of resellers (Google carprouk and the site will come up).
Its usually £12 for 100ml (which makes 400-500ml when diluted), however, if you only want it to do your helmet, I'm sure he used to do a smaller sample bottle at 50ml, which would be more than enough to do your helmet a few times
EDIT, yeah, he does a 50ml sample for 6.50, unless you want to do the bike as well, you probably wouldn't need the 100ml.
If your looking for the cheapest place to get some, your best buying it direct from carprouk rather than one of the myriad of resellers (Google carprouk and the site will come up).
Its usually £12 for 100ml (which makes 400-500ml when diluted), however, if you only want it to do your helmet, I'm sure he used to do a smaller sample bottle at 50ml, which would be more than enough to do your helmet a few times
EDIT, yeah, he does a 50ml sample for 6.50, unless you want to do the bike as well, you probably wouldn't need the 100ml.