Don't know about Schofield and Sims, one of my to do list things for later in the year is to buy the mental arithmetic books recommended on here. Aim would be to start building up speed from about Jan time year 5.
These are pretty good and a few people have recommended them on here. They cover a wide range of topics and include varied styles of questions. Bear in mind, though, that the workbooks do not include answers, you would have to buy answers book separately (they are not expensive). The answers books are identical in layout to the questions books, but simply have answers shown next to each question, so if you wanted to work around it, you could get away with just the answers book.
If you do go for S&S, you will probably need books 4 and 5, or just 5. Book 6 has quite a lot of secondary school stuff in it.