Thanks for all your suggestions, I may seek a second opinion as to whether or not I have to totally fill the hole.
I have a second old hole, with a much better manhole, and was told that was safe to leave, and perhaps I could coax them out of one into the other - failing that I'll leave them where they are and try not to jump up and down on hole 1!

This new garden has the worlds largest slugs in it, so there is plenty to feed on ...