Could ‘Scratchbury’ come from ‘Old Scratch’ – the Devil? I mean he was about the area, having made Cley Hill, so it’s not inconceivable he might have sat here.
The Oxford English Dictionary has an example of this euphemism’s use from 1740 – but its older forerunner ‘scrat’ and other similar words mean all kinds of gobliny devilish things in Old Norse and German.