I don't know if you have found these sites yet.
I went to the Motorsport Memorial website to find further details, such as where the accident occurred, and they have no record of Ralph Miller being killed in a Lotus Eleven. They do list a Ralph A Miller but say he was killed driving a Zink .
The Racing Sports Cars site also lists Ralph A Miller and lists him racing several cars including Lotus XI #150, Lotus XI unidentified and a Zink Mk 3, which suggests he is the right man. Unfortunately they do not have photos of either car - or in fact of any car driven by him.
Martin Krejki's WSRP site lists Ralph Miller driving Lotus XI #150 at Sebring in 1956.
So, it looks as if your grandfather raced Lotus XI #150 and might also have raced #153 but was killed in a Zink rather than athe Lotus XI.