I would try advertising for litter mates the same way that you found Charley, ie was he ad in Tesco, Free ads, local paper, ad on PAH notice board, ad in newsagent etc? That way you may catch the attention of the people who looked in these places earlier this year. Would it be worth contacting local vets / dog training classes too? saying you were looking for litter mates, please could they pass your details on etc