If they cannot obtain minibuses and coaches their website should say:
Unfortunately we cannot run a replacement road service because we cannot obtain buses and coaches to do so at present. We are continuing to investigate options for this evening's peak and would appreciate your patience; we will advise as soon as we know what we can do. If at all possible we would strongly advise you make other arrangements, such as being collected by car from Wellingborough. We regret refunds cannot be made in respect of individual taxi fares incurred, however <fare refund policy goes here>.
What it does say is:
Which is patently false.
Well, actually it probably isn't.
First things first - to get to the M1 going via Wolverton is a very long way around - easily adding 40 mins to the journey.
As for the non-motorway roads - I can tell you for a fact (because I came in to MK on the B526 this morning) that there was ALOT of flooding on various roads - to be honest, in the dozen or so years I've been doing the journey I've NEVER seen anything quite like it with rain - the only time I've seen anything close was when it snowed about 10 years ago.
Long Buckby station is very isolated - so running to there would mean running via lanes which are flooded.
And at Northampton the A4500 - the main road into Northampton from Jnc 16 is closed due to flooding - which if you bother to look at Stagecoach's website you will see they are having to cancel and divert buses.
I assume that with your righteous indignation you have experience of arranging transport at short notice to know what is or isn't achievable?
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Anyone who doubts what road disruption there is around Northampton / Northants at the moment due to flooding need only check here:
http://www.bbc.co.uk/travel/northampton/incidents/road