Wonderful, thank you both - not sure if I can get RainX this side of the pond, but I will look for an alternative and give it a go. I've certainly found that a hat compounds the issue, though can be useful for reducing glare. Unfortunately I'm really rather unfit for my age, so heavy breathing, sweating and cursing is sadly par for the course when birding in the tropics. More exercise and rainx before December I think! All the best and again apologies for the slight tangent.
Kind regards, Daniel