David, whilst I have never had bedlinen as heavily soiled as you describe, a member of my family has trouble with bowel control and produces similar, stains I would say when it comes to underwear. I have to say that we've used Persil and Ariel for the past few years and both products haven't left stains or odours at all... have never had a problem with Persil leaving stains or odours behind myself.
I agree with the sales figures though, sorry Rob, but even though this is from a checkout boy's perspective I scan probably 3 packs of Ariel (whether it's powder, tablets, liquitabs) to 1 pack of Persil. Ariel is always being replenished more too from what I've noticed and always runs down faster so Persil doesn't outsell Ariel at all at least round here.
What I can say is that, I use a colour detergent for everything that's not white, and repeatedly Ariel has always outperformed Persil Colour. Ariel Colour always gets grass stains out of jeans (even the liquitabs), whereas a dose of Persil Colour powder doesn't touch grass stains at all. So I feel Persil lags behind when it comes to colour care products, and is nowhere near Ariel unlike the traditional biological formulations.
Jon