Science fiction about aliens in a mine

I read this book in the 80's but I think it predates that, and may have been one of the Gollancz "yellow cover" series. It's also possible that I have mixed up the details somewhat... it was a long time ago. The main character is called in to investigate a disturbance in a mine in South Africa; it was probably a diamond mine - very deep at any rate. The miners report seeing ghostly figures that rise up out of the floor once a day and then sink back down again. The miners are a superstitious bunch and refuse to work. The main character arrives on site and is able to witness the ghostly figures - they turn out to be aliens, not ghosts. Their planet is slightly smaller than Earth and occupies a different dimension - so that the aliens appear transparent and ghostly. Their planet used to share the same location in space as Earth and was therefore completely concealed - but something has happened to disrupt the orbit of the alien planet, making it slightly eccentric to the Earth's core - so every night, due to the non-aligned centres, the surface of their planet rises relative to the floor of the mine. The main character is able eventually to make contact with the aliens. For some reason he decides that he wants to transfer over to the other dimension. This is difficult but eventually they work out how to do it - only his consciousness can transfer and he must inhabit an alien form. Once there, he meets another alien who also claims to be a human consciousness. The main character has to pass a test that involves identifying what sort of alcoholic drinks should be served in which type of glass - something only an original human would know.

I've been searching for this book for a long time on Google with no result. Please help !