>...maybe the pigs were put in the ditch to be picked off?<

Well, that's what I've been sayin' all along but no-one believes me ;-)

The ditch at Avebury might have been used as a mega-circular pig run. Dudes would stand on the bank picking off their own (and maybe other people's pigs as well ) in a ceremonial Pig-Pop followed by a ceremonial Porky-Fest. Makes sense, and I'm sticking with the idea until a better one is advanced for the Avebury-type bank and ditch construction :-)

As for a good purple - difficult and expensive to obtain (hence its association with royalty) but I guess you could approximate the colour with combinations of blue and red.