More of a Fleet Switch Suggestion but wouldn't it make sense for Stockton to Swap:

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for Darlingtons:

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That way Stockton has a Extra Couple of Euro 6 Vehicles for the X12 and Darlington has some more reliable 61 Plates just in case the E300's all start have major issues due to their age. I always thought it would be beneficial to both Depots. Just a thought. Also brings the older buses to Stockton / Redcar Area to make it more appealing to get a new big batch of vehicles in the future.