It says that the British Association for the Advancement of Science had a write-up of it in the 1880s. But that doesn't seem to be online. Which is a shame as it'd be nice to hear what they thought. And it'd be interesting to know what type of rock it is, whether it matches other glacial erratics supposed to be in the area. I liked Chance's comment that when it's wet it looks very red and maybe blood-like?
Very interesting item on St. John's stone included here. Pastscape say's that the area around the stone got used as the city's rubbish tip before being redeveloped for housing.
The article also mentions "part of St. John's Stone now sit inside St. Luke's Church in Stocking Farm, Leicester". I also note that "there were alter stones at Barkby on Ridgeway, in Markfield on Alterstone Lane and in Parker Drive, off Blackbird Road"."There was also something in Enderby, she said, an old mistletoe bush worshipped by the druids."