Jeff Layman wrote:
Go to your local pharmacy and get some loratidine or cetirizine
antihistamine tablets (these are "non-drowsy"). This will help dampen
the response to the bite. Also get some hydrocortisone-based cream to
rub on the bite.

It took a few days before realising what it was. It felt like I'd walked
into something and it had bruised and swollen. I've been putting anti
histamine cream on it the last 2 nights, I would have taken an AH tablet but
I think they have accidentally gone to visit my mum in the boys' suitcase.

As you react so badly, you might want to consider using insect-repellent
spray or cream before venturing into the garden for any length of time.

It only happens once or twice a year, and tbh, I don't think about it until
it occurs. Mosquitoes are annoying, but don't cause anything like that
problem (they did when I was pregnant and in Italy, though!)

From previous suggestions, the most likely culprit to my mind is
http://en.wikipedia.org/wiki/Blandford_fly

My next door neighbour suggested red ants, Nick thinks it's a spider (but he
would, he's a little spider-phobic)