Don't know, but Bustinskin organise a pretty tough one in late September. Swim in Portland harbour, then bike round Portland, out into the very beautiful but distinctly unflat West Dorset countryside, followed by a tough run round Portland.
I've not done it, though a couple of pirates did it last year - one described it as "a brutal day out". I watched. The first runner back was part of a relay team, so had only done the run. As he finished he said to the marshalls "That was ***king ridiculous!"