Backdated valuation needed on property gifted, not sure of date to use

[TessaW]

Father died in 2002 with everything left to mother. No inheritance tax. Mother was advised that she could do a Deed of Variation and use father's nil rate band to transfer his share of their home to me and brother. This was done, together with a trust deed and the property transfer. No valuation was done at the time as it was clear that 50% of the property was well within the nil rate band. Now realise that a valuation should have been done and not clear whether it should be done as at the date of father's death or the date when the legal matters re the property transfer were done (a year later). Please can anyone advise?