I have the same understanding as Whirlybird regarding the class 1 to class 2 validity. Class 1 expires but degrades to class 2 level for the remaining validity period of a class 2.
I was at Gatwick earlier this year (Class 1 renewal) and the AME reminded me of the 5 year renewal period (to help me save money). I suspect the five years starts ticking down from the date of the last Class 1 exam.