At a guess, First Group are currently short of serviceable coaches to operate on the X1 Lowestoft/Great Yarmouth to Norwich, Kings Lynn & Peterborough route, so they've simply leased a coach from Stagecoach (who've got some of their 'Megabus' fleet available after they've cut out unprofitable routes). i.e. what you're seeing might be just an ordinary X1...
The Megabus website refuses to accept Great Yarmouth as somewhere that you can book to or from but it's shown on their route map here:
http://uk.megabus.com/Route%20map.aspx
Seen today at about 12.40pm, but not close enough to notice passengers or empty. I couldn't find Gt Yarmouth on the site either, how would you book that Route? I can't see the route map on my ipad.
At a guess, First Group are currently short of serviceable coaches to operate on the X1 Lowestoft/Great Yarmouth to Norwich, Kings Lynn & Peterborough route, so they've simply leased a coach from Stagecoach (who've got some of their 'Megabus' fleet available after they've cut out unprofitable routes).
i.e. what you're seeing might be just an ordinary X1 service but operated by a vehicle leased from another company.
Sorry, we can't find any related questions. Try using the search bar at the top of the page to search for some keywords, or choose a topic and submit your own question.