Thanks Towger and Vanilla.
I think (I hope!) that at least some inheritance tax was paid. However, there's now a difficulty because the solicitor messed up the inheritance tax declarations, so the tax due is different. If the newly calculated tax due is less than what was paid, will it still be possible to claim back the difference if it goes past 4 years from a particular timepoint, and if so, what will that timepoint be? Will there have to be a new valuation date when the new inheritance tax self-assessments are submitted? I see the valuation date on the original forms submitted was the date of probate.
Also, are there any other possible implications of the long delay in finalising this estate (i.e. currently almost 4 years from death and 2.5 years from grant of probate/valuation date)?