Complex sentence: Simpson's, once amongst the United Kingdom's leading clothing stores, opened on Piccadilly in the 1930s, but the store closed in 1999; the site is now the flagship shop of the booksellers Waterstone's.
Split into simpler sentences: Simpson's, once amongst the United Kingdom's leading clothing stores, opened on Piccadilly in the 1930s. The store closed in 1999 and the site is now the flagship shop of the booksellers Waterstone's.