Offer to obtain evidence

At a child DLA hearing a novel point was raised by the doctor.
This had not been anticipated and so no evidence had been obtained.
The mother gave evidence but this was insufficient for the panel.
The rep suggested/offered that evidence could be obtained from the consultant.
The Panel did not take up this suggestion/offer - no reasons given.
An adverse decision was made.

Someone mentioned there was a case where this was held to be an error of law.
Anyone know or any further advice?