Went to the garage today with the Harley Benton and I can now confirm that Yes, a Harley Benton does fit into a Volvo C30 It was like the boot opening was made for it as there was about an inch clearance each side!
I have a 2005 Mazda 3, which I imagine is a similar size and if I fold down the back seat I can fit a Mesa oversize 4x12 cab, hartke 4x10 cab, two guitar hard cases, and two amp heads.