HJ
The time to request an MR is defined in law, there is no flexibility but that does not mean that you do not have options.
You can make the request in the one month normally allowed, you could then ask the DWP to wait until the information is available. They normally give four weeks although this includes postage, I can't say whether they would give you longer, however, Decisions are currently taking 16 weeks this may give more flexibility.
Or, you can make a Late request for an MR when you receive the information, you will need to explain why it is late, waiting for the documents should be accepted as Good Cause.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
