Thanks for the swift reply.

Last booking was a one-way fully flex economy fare NCL-CDG-ALG with AF. This time my booking is a mix of AF / KLM (NCL-CDG-ALG, MAD-AMS-MME).

I managed to choose a seat in the Business cabin of NCL-CDG (so maybes not technically an "upgrade"?) which cost me £54 I think.

There was the option of a definite upgrade on the next leg but I didn't bother as it was €400+.