In reply to Russell Lovett:
Ideally you want an area of high pressure or anticyclone above relatively cold air at the surface leading to the formataion of fog or very low cloud in the valleys. The best conditions tend to be around dawn as daytime heating from the sun can cause the cloud to lift and break up, although this time of year there is not a lot of strength in the sun so cloud inversions can be quite persistent.
For the Lakes, the best conditions often occur when high pressure is centred over south/central England as this will give a westerly airflow across the Lakes (wind moves clockwise around a high). With a sea track coming across the Irish sea the lowest layer of air will pick up moisture, helping low cloud to form. Bare in mind that the height of the inversion tends to rise away from the centre of the high, making it more likley the cloud tops will be above the summits.
