The Winter Ops manual does contain this in 14.2:
"In order to protect our normal schedule of flights, Airfield Operations will closely monitor the operational capability of UK Airports and determine the number of parking stands available for diversionary aircraft. This assessment will be made at intervals of not less than 12 hours between the months of November and March inclusive.
The ADM will set an “inbound diversion cap” and notify ATC of any capacity limitations. The “cap” is intended to identify the number of inbound aircraft which can be accepted without impacting upon stand allocations plans associated with our normal schedule of flights. In the event of a mass diversion scenario, the ADM and TDM will activate Diversion Control in the CMC.
...
In the event of significant disruption at Manchester Airport and limited availability of parking stands, a decision may be taken not to accept inbound diversions. Requests from Manchester Airport airline customers to accept inbound diversions and extra flights will be considered on a case-by-case basis by the CMC, subject to the provision of a Ground Handling service."
ADM= Airfield Duty Manager
TDM = Terminal Duty Manager
CMC = Crisis Management Centre