Set in England, a single mother has two children older girl and young boy. She is approached by a man (a lawyer) who asks the mom to apply for a job as a caretaker of larger estate. The mom goes to the law office, and sees the man’s picture on the wall, who has been dead for many years, but looks like the current lawyer (blah blah blah the 3rd). Anyways, she gets the job, and the family gets to move in.
In the garden, the two children see ghosts, a boy (maybe George? maybe Samuel? ) and a girl (Sarah) from the past. Tom was the gardener’s boy Sarah was in love with when she lived. They ask for help as they were murdered (drugged and then the house burned). The lawyer (the ghost who approached the present day mom) was their uncle who had ignored their pleas that their guardians were going to do them harm.
They make a potion and go back and time and help. The present day kids don’t know if they did save them , until the present day lawyers visit the mom. Turns out, her young son is the direct heir to the estate! The ghosts did survive, Sarah married Tom and their decedents were the kids they asked for help! The little boy from the past never approved of the marriage, moved to America, and never had any kids. I believe ‘garden’, ‘ghosts’, or ‘time’ in title.
Probably The Ghosts, by Antonia Barber.
The Ghosts is the US title. You may the book under the title The Amazing Mr Blunden.
I’m sure this is “The Amazing Mr Blunden”, but there are also ghosts called Tom (a kitchen boy) and Sarah (the daughter of the house, but from an earlier time) in another book published in Puffin from the 80s, “Moondial” by Helen Cresswell. (I revisited “Moondial” not long ago and was surprised to discover how moving the story is from an adult point of view – as a child I just read it as an adventure story!)