I have just had a response from Shopify support which fully explains this situation. Basically a change has been made recently that limits the number of real orders that can be placed for a development store (orders created in the admin section of the store are classed as real orders once marked as paid), once this threshold is reached, checkouts are disabled across the board, including via the Bogus Gateway and Shopify Payments Test mode, so the workaround stated in this thread won’t work.
You need to contact support and ask them to raise this limit, which will enable checkouts again and allow you to use either the Bogus Gateway or Shopify Payments Test mode. Be warned, they only increase the limit by one, so if you create an order in the admin section of your store, the order limit will be reached and checkouts will be disabled again.