Krallu
I hope you can clearly see what's on the leading edge in this photo. Having expected it and noticed it, I asked my friend in the R/H seat what he could see - nothing. Take another look and he realised what was happening. Cruising above the cloud over Wales and started descent. Pitot on in advance and this started as soon as we entered cloud. Surface OAT was about +4, at 5.5 it was about -7 deg c. Both Altimeter and VSI failed for a short time. This was Feb.
Don't stay in it if you're not equipped! Been in similar over North Wales in June, so don't think it can't happen in summertime.