What happens now?

Grandmother leaves in Will £100k (invested) with income to Daughter for life and then to Grandaughter (age 50+ and disabled). Money still invested as Will Trust of Grandmother. As Daughter died 3 years ago what is a) position regarding reporting income/ gains of the investment? Presumably Grandaughter should declare? but Trust Return is being completed.

Assuming the original estate administration was concluded (and it sounds as though it was), the granddaughter has been absolutely entitled since her mother died and should have been reporting income/gains personally rather than the trustee(s).