In City Facilities Management (UK) Ltd v Ling (2014) UKEAT the Employment Appeal Tribunal (HHJ Eady QC) has reviewed the approach to be taken in relation to expert evidence where the issue of disability for the purposes of s.6 Equality Act 2010 is in dispute.
The case is useful as it reviews the authorities relating to expert medical evidence and notes in particular the approach in J v DLA Piper LLP UK  ICR 1052 whereby it is suggested that the Claimant (who bears the burden of proof) sets out the effect of their contended impairment on their ability to carry out day to day activities. This is the starting point. It may be that expert evidence is subsequently required, particularly if there are issues of credibility.
The Employment Appeal Tribunal does not appear to have definitively held that a Tribunal has no power to order the Respondent to pay for the full cost of a jointly instructed expert, though the Judgment indicates no particular desire to go down that route either.