It's called a "water evacuation drip moulding" part #8248 .71 - we got ours on-line, from (I think) Guy Perry Ltd., £22.42
Their parts enquiry page is at http://tinyurl.com/ccgmv2x
You can buy one from any Peugeot or Citroen dealer. They call this part a "water draining gutter", part No 824871, current price £19.94 inclusive of VAT.