DALR is 3C/1000ft, but dewpoint lapses at 0.5C/1000ft as Heston says. Subtracting one from the other gives you the 2.5C/1000ft lapse rate
As Prazum says, (temp-dewpoint) * 400 gives a good answer. You can replace temp and dewpoint with max day and min night temp from the BBC forecasts to get a view of what's going to happen in the next day or so.
I also found a little formula that says...
Cloud Temperature = -0.003 * cloud height (feet) + air temp (C)
(note the minus sign at the beginning)
I can't say whether it works or not (anyone know?), but for clouds at 5600 feet and a temp of 21C on the ground, this calculation predicts the cloud temp will be 3.8C, which means IMC rated pilots could go up into it without too much fear of icing.