Hi Dave,
As HD says, Heathrow continue to operate on Westerlies with a 5 knot tailwind component. Also there are many occasions when the wind direction varies markedly between the two airports; I do not know the meteorological reason for this, but it is just a fact of life. Furthermore, some companies at City cannot even accept 1 knot of tailwind component, given the limited runway dimensions. It is a regular occurrence on days when the wind is variable to change the runway in use at City in the middle of an inbound sequence. Hope this helps.