Yes, Kris Marshall stole the show from them quite a bit during the first four/five years, but without the performances and chemistry of Robert Lindsay and Zoe Wannamaker 'My Family' wouldn't have been the show it was, or ran for half the time.
This episode perfectly shows their chemistry, and you at this stage totally beleive that Ben and Susan love each other. Towards the end you did wonder why on Earth they were still together.
There's a slight but nice subplot too, involving Nick trying to 'parent' Janey (who is made to look absolutely gorgeous here) and Michael in their absence.
With a sharp script and some great twists, this was enormous fun.