Thank you for the ideas @mrssunshinexxx
As odd as it sounds, I don't want to interfere with mum's ashes, so wherever she goes she goes in totality. I like the idea of jewellery (considered it for my cats ashes) but that item would then become the most important thing in the world and I don't want the burden / responsibility of keeping it safe. Sadly we aren't in our forever home so the garden isn't an option.
I actually spoke to my dad today and we are going to go with plan a. Mum is going in the garden of remembrance, back to nature.
I'm going to get something custom made (at some point) with Amber my cats name and mum's birthstone. At least if that gets lost or destroyed it isn't anyones ashes.
@Brillig I know people who have family outings to the cemetery!!! My mum would turn in her metaphoric grave if I had my kids at the cemetery.... she'd want me to take them somewhere fun.
I think my MIL was a little bit perplexed about my mum's funeral. That side if the family are used to big, loud, long funerals (they are Irish) so our quiet, very small and intimate service was alien to her. She couldn't understand why no kids were invited??? 
My MIL is actually a bloody lovely woman, she has been a huge source of support, comfort and love to me and I know my mum was happy in the knowledge that me and the kids are so well looked after by her and the rest of DP's family.
Its just weird how different we all "do" death?