Extreme is meaningless - it's only relevant in relation to perceptions of the centre, which keeps moving.
My benchmarks were set in the 1970's and 80's and seemed fairly straightforward. The centre was mid way between Labour and the Conservatives. Now, Corbyn, who is a pretty mainstream socialist is called 'far left' and the Conservatives who are doing things Thatcher (who was seen as 'somewhat' right wing) wouldn't, and are being called centre-right !
The world's gone mad...
But if the centre has moved more right over the years then naturally what was once considered moderate left might now be considered far left. When Labour won in 1997 it was because they moved closer to what was considered the centre ground at that time.