The alternator drive belt tensioner had failed and the belt chewed up.
Have ordered like for like Gates tensioner via ebay but unless I go to Peugeot I'm struggling to find the definitive correct belt and idler pulley.
Weirdly, even the dealer couldn't tell me what tensioner it was from the part number stamped on it despite the fact it had Peugeot and Citroën logos and was OE the part number on there is apparently a Gates number and was not the part number PSA use for ordering!
Bonkers!
Have to get the idler off to get numbers and I'll find it myself but because the belt is wrecked beyond recognition to lift numbers that'll have to be dealer £££££££££ !
I'll try and get pics for a how-to if I remember.....