Hi modeste diesels don't like the cold have you tried turning the key on and off 2 or 3 times each time the pre heat light goes out to get the glow plugs good and hot before you full turn into start mode hth tez
The heater plugs can and do wear out, they are not too complicated to replace, also check your fuel pump is giving the right pressure. I would take it to an independent garage if you need checks done as most peugeot mechancis couldn't walk and chew gum at the same time!