I’ve been bitten by a little diamond-shaped bugs while doing gardening both in Russia and in UK. The bite hurts, then swells and itches quite a lot. I had no idea what this insect was so I searched the images. The first finding worried me a bit:
American blood sucking Kissing Bugs (members of Triatominae) transmitting Chagas disease. My granny often had swelling around her eyes while gardening in the spring which looked just like the early stage of that disease. I hardly could walk for a couple of days once when I had several bites on a foot at once.
Then I found another Reduviidae bug, native to Europe and mostly harmless: Reduvius personatus or the masked hunter.
There also are similar looking Damsel bugs.
That probably was it. Nasty bite but no worries. Keep gardening.