I would ask SL to explain how the tax charge arises.
The tax position depends on the type of lump sum paid, what kind of scheme he was in and the age of your husband at death as, as others have said, whether your husband's total pension pot exceeds £1.5m.
You say your husband was 56 and because you refer to Scottish Life as the provider, I am guessing this was a personal pension. The only question left is what kind of lump sum it is. There are various technical kinds defined in tax legislation and they are all treated quite differently.
If the lump sum is an "annuity protection lump sum" (which might be the case if your husband had already used his pension fund to buy an annuity), the rate of tax is 55%. However, if your husband was not receiving any benefits at the date this type of lump sum may not be relevant.
Where no benefits were in payment from the policy, the lump sum payable would usually be a "uncrystallised funds benefits lump sum death benefit". This is taxed at 55% if the member was over age 75. However, where the member is below that age, the lump sum should be tax free (unless the £1.5m point is relevant).
You should go back to SL and ask for more details and if they still say tax is due, consider taking your own advice if you are not happy with what they say.