I've had this problem before with Northern (used to be a cycle courier and I'd commute into the city by rail). If there's an RRB/alternative transport provision in place, despite the website saying they encourage cycling, Northern's official stance is 'it's at the discretion of the transport provider' - ie 'not our problem' and they effectively absolve all responsibility - you really do have to just go and find a guard or someone that will authorise an alternative as a courtesy (if there is one), and hope for the best as they genuinely will just abandon you if there's no solution. It's a joke really. Not even 'check before you travel', the official advice is basically to gamble.
Generally, the RRB providers refuse to carry full size cycles - I've had the odd one where it's been a coach and the driver has allowed it in the hold (though admittedly, this has been when there are next to no passengers or luggage), but if it's a service bus (ie a single/double deck, not a coach) they'll pretty much always say no (had a TPE one recently where a passenger with a dog became aggressive as the bus driver had put a hand written note on the door stating 'no cycles, no food, no dogs' and refused to let anyone board until he left). I think I've heard of even folding bikes being refused sometimes.
Once a few years ago on my bike, I was met with a bustitution at Manchester Victoria - this was when Northern still ran the Huddersfield stopper, it was late in the evening (~11pm) and there were no staff in the station, and no alternative services. A local Stagecoach E400 was the replacement - the Stagecoach driver was uncertain if he could carry the bike, so called his depot who said it wasn't permitted, and instructed him not to allow me to travel.
The Northern transport coordinator proposed that I leave my bicycle in Manchester and board the RRB without it, and failed to see why that wasn't appropriate advice. I managed to travel thanks to a sympathetic coach driver that could see things from my perspective, but if it wasn't for that I think I'd have been making a rather substantial complaint...