Read in 1960s, aged 7/8.Set in the then present and Victorian UK. Mustard & white hardback cover with black line drawings of an ornate mantel clock with dolphins/town house. A boy and girl found/bought the clock in the summer holidays and traveled by it to live with a family in Victorian times.
Addition: When I was 7 or 8 in the 1960s my dad read me a hardback library book which neither of us has forgotten and which we mention fondly now and again. My dad is 90 on 13 June and I want to find the book as a gift for him. I do not know the title or author. I remember a pale brown//yellow and white cover with a line drawing in black of a gilt clock and perhaps children. It was a book about children of the time (i.e.60s), a boy and a girl I think (I imagined them to be the same age as I was), who were given/bought a mantelpiece clock with ornate dolphins adorning it from a junk shop. It was the summer holidays. Through the clock they were transported to Victorian times where they lived as part of a Victorian family and had adventures. They found the Victorian clothes, etc. very strange.