Complex sentence: Walthamstow in 1910 was a very different constituency to when it was created in 1885, the expansion of London having changed it from a rural area of Essex to a mostly urban suburb of the capital.
Split into simpler sentences: Walthamstow in 1910 was a very different area to when the constituency was created in 1885. The expansion of London had transformed it from a rural area of Essex to a mostly urban suburb of the capital.