I’m haunted by a (probably YA) time travel book I read in the mid 70s. I believe it takes place in England, where relatives are visiting (maybe from America?) their professor/scientist Uncle (?) who has produced a time travel potion. I think one of its main ingredients is monkey brains, but I’m not 100% sure. The serum allows the imbiber to travel back to Middle Age Britain, not sure if it’s as an active participant or as a fly-on-the-wall. Whichever character it is that starts taking trips, soon gets caught up in the people and events of that past. He becomes addicted to it. The danger is that his body stays more-or-less in present day geographically, so when he comes out of the trip he may be standing in the middle of a super highway or some such that wasn’t there hundreds of years ago. I don’t remember the resolution. I just remember loving the imagery and concept. Anybody?