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You'll normally find them on school services mainly but as its the school holidays they're getting out and about a bit more. Usually the 13 is the best place to find them almost guaranteed at the moment. Others tend to appear when something breaks down so it's harder to predict where they'll turn up, but services 12, 39 or 52 and 58 (both of those interwork with 35 every few hours too) are usually good bets to find one out and about.