Hello Merlin,
More often than not decisions for PIP and ESA are made without the claimants GP or Consultant being contacted. Tribunals will usually make a decision based on all the evidence they have in front of them from the Claimant and the DWP and where it is an oral hearing from speaking with the claimant. However it has been known for the Tribunal panel to adjourn a hearing if they feel that more medical evidence is required, but once again this isnt the norm.
Nothing on this board constitutes legal advice - always consult a professional about specific problems