This stuff is everywhere, growing out of cracks in walls. The tiny mauve and yellow blooms are no bigger than a fingernail. As you can see from the pics below, it seems to need nothing more than a bit of grouting to fasten onto, and spreads across otherwise barren surfaces, making them beautiful!
I started my ill-informed Googling by trying 'minature pansy creeper' but got nowhere till I looked more closely at my photos and realised that its leaves looked a bit like ivy - indeed, ivy was often growing on the same surface. Bingo - it's Ivy-leaved Toadflax!