Holly wrote:

Does anyone have any ideas of what I could put inside the shoes to give
more friction but without being sticky? Got some insoles but they are a
bit thick so make the shoes a bit tight and in any case are only
slightly less slippery. I wondered about a thin smear of clear
silicon/acrylic allowed to dry. Any other good DIY ideas please?

If the sticky suggestions don't work, you could try attaching heel straps
made of the clear, thin, flat elastic often used in lingerie and
swimsuits. It's nearly invisible, and should help hold the shoes on your
feet.

Sheila