I think it was in the latter half of the show. I have just listened to it on iPlayer.
They were discussing Notts and another force getting rid of their mounted section. Plenty of pros and cons, but I think the main reason is expense, rather than that they are not effective.
When they are not helping to control demos or riots, they are patrolling streets and making publicity visits to schools etc.
Also, not discussed is what would happen to the horses?