In reply to James Paul Robinson:
It's hard to be sure, but anti-histimines seem to work so long as they're taken before the bites have festered overnight - there appears to be a grace period.
I'm prone to being midge-bitten and left untreated get insanely itchy, tumescent pustules. But, whenever I've had a day of being bitten and remembered to take an anti-histimine before bed and for the next couple of days, I've been okay - the bites haven't swollen or gone itchy.
That said, it's impossible to know what the outcome would have been if I had done nothing! But, it seems well-worth a shot for fellow sufferers as own-brands pills only cost around a quid for seven days supply.