Aggie , here in the West of Scotland you need permission from the council, and they’ll open a bit of grave and charge you £500!
We dug up a bit of the earth around my parents’ gravestone, and sneaked my brother’s ashes in. We planted two hebes either side of the stone, and one flourished, but the one where the ashes are died very quickly!
And not at all comparable to your mum, but we had our dog’s ashes in the house for a few years, as the Bodach couldn’t face scattering them, and always said he’d go with whichever one of us died first, so he accompanied the Bodach , and I found it really comforting.