The slider you can move to compare the temperatures across the country between the years doesn't appear to work using the link in my OP.

I don't know why. May be because a missing # in the link.  Try this instead:

http://metro.co.uk/2015/12/20/its-30c-warmer-today-than-it-was-five-years-ago-5576310/#

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CD - We won't know until December is over, but according to XC weather on my mobile it's still predicted to be in double figures for the next week (down south that is).