In the end I gave up and asked the shed supplier to put down a base and they used a dry mix and some slabs with a minimum depth of dry mix of about 30mm.
Base seemed very good and with the shed now on it I am pleased I got a good stable base as the shed is nice and true and feels very well supported and stable.
Base seemed very good and with the shed now on it I am pleased I got a good stable base as the shed is nice and true and feels very well supported and stable.