Once again Sharon Lawrence has turned up trumps for us. 
Stagecoach is taking ADUK branding off their buses and soon Megabus will as well. That's a UK-wide change!!

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Due to the complexity of this case, I consulted with my senior colleagues and I have this afternoon contacted the MD of Megabus as per their instruction.

Bus Users have asked Megabus to take a look at their Conditions of Carriage: Clause 10 and to amend this clause to permit all assisted dogs to be permitted travel, we do hope that we will be able to resolve this matter for yourself and other affected passengers.

Once your case has been dealt we will raise the issue with other providers who may have similar T&Cs

Complaints Handler
Bus Users UK