Of course a private lease isn’t going to be cheaper than Motability as they don’t get the benefit of the VAT fiddle.
The private lease route is only better that it allows you to choose a car you want if it’s outside the remit of the Motability restrictions ie. cost and bhp.
It’s very true what BionicRusty says in that the lease deals are very fluid and cars for whatever reason can suddenly reduce massively in price. You can also get some cracking deals on ready spec’d cars some of which come fully loaded.