I recall my ride on BF63 HDC during its X21 trials, from Chester-le-Street to Durham.
It didn't have much oomph as I recall, and felt a bit like an Angel B5HL, but the engine was a lot more noticeable.
I'm no expert, so I really can't pass comment other than - I think they wouldn't do bad on the X21.
Go North East: 2014/15 Financial Year Order Predictions
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