During numerous visits to the London hospitals, I have found the apostrophe comes in many guises: Guys, Guy’s, Guys’  – poor Guy must be turning in his grave.
St Thomas, St Thomas’s, St Thomas.’
St Thomas is interesting. The ‘s’ is present in the name so presumably the apostrophe would go after the ‘s.’ But, and here’s the rub, should an ‘s’ be added, hence St Thomas’s? After all, it is usual to write Margaret Davies’s house.

I believe David Crystal has said that as Jesus is universally and historically known, no extra ‘s’ is required, hence Jesus’ disciples.

Does St Thomas have similar status?

In any case, whoever is sitting in an office at St Thomas’ or Guy’s Hospital, please apply some consistency.

A frustrated patient.