Upvote:4
Apparently, these maps are always available online, but are hard to find except when someone toggles the "disruption" flag on the website, which doesn't always happen quite as quickly as you might like...
For Thameslink routes, the list of stations and their associated alternatives are here on the Thameslink site. For the semi-linked Great Northern routes, they're here.
These maps include alternate rail routes, alternate bus routes, and instructions on how to get to where you've going via those alternates. For example, for Luton Airport Parkway (for those going to/from the airport), the map looks like this:
And includes instructions on what buses to take to get to Bedford, what bus to get to alternate trains to London, and details on how to find the stops for those buses