CGT jointly owned property discount

Me and my brother inherited a dwelling 50:50 from our mother when she died a number of years ago. The estimated value at the time, plus the small amount of cash left, was significantly below the inheritance tax threshold. No probate was obtained, so no probate value declared at the time. We are now selling the dwelling. Neither of us have lived in the house continually since inheriting, so neither can claim 100% private residence relief. Our advisor has stated that because the property was inherited jointly HMRC apply a discount to the probate value used as base cost in a capital gains tax calculation. This is to reflect the fact that a half share of a jointly owned property is worth less than than a pro rate value of the whole because of the difficulty in selling a half share. I am unable to find this in any HMRC guidance. Could you advise where the guidance is? Thank you.

Hi ,
 

This is at : IHTM15072 - Valuation of joint property: discounts for joint ownership   .

Thank you .