Just pass them on, it's the governemts job to see this doesn't happen.
I got passed one by my old landlady(in change), but I spotted it, spoke to her handyman that works on her properties for her, and she tried him two days previous with same note, she's a JP and her son a copper name of Durrant, just spend them is what I would do, can't say I've ever known the plod being called, might be worth seeing if a bank would exchange them for proper notes ?
tabetha