Valuations with shared ownership and sitting tenant

I am negotaiting with the DV over the value upon acquisition of a property which my clients have now sold. Firstly, they are applying a 10% discount for joint ownership (husband & wife) and also a discount re the sitting tenant - an 89 year old lady who moved out and died very shortly after my clients acquired the property. In this latter aspect the DV is discounting for 4 years further life expectancy despite the fact she did not live that long.

Does anyone have any experience of successfully arguing against such discounts and in particular re the sitting tenant.
Antony Pearce