I'm getting a 'couldnt save entry' error for a single but all required fields etc are filled
Under your single's section settings, you had an invalid URI format for the cy_gb (Welsh) locale: ein heiddo.

URI's can't have spaces in them.  It'd be nice if Craft could warn you that the URI format is invalid (no spaces, all URL safe characters, etc), but it's difficult because Twig code is allowed in those fields and all sorts of characters that aren't URL safe are allowed in Twig markup.