As long as the pilot is visual his best means of detection are his eyes. most of this week above 5000 feet there has been very little cloud over a large portion of the UK and below 5000 feet well broken cumulus with suoerb visibility above.
Early posts on this thread appeared to establish that the far-travelling ash from that phase of this particular erruption was effectively transparent to both visible light and the radio frequencies used by weather radar. It did reflect, however, at some infra-red wavelengths. So despite the visually clear skies, the nasties were there and the Mk1 eyeball was unable to detect them. The first D-CALM flight reported this - also saying that it avoided flying into stuff that could be detected only by its special atmospheric research kit.