Thanks for the feedback - haven't been on here in ages so sorry for the lateness of the reply.
Since starting this thread, I've run two sessions, and the combats in both were at opposite extremes. In the first session the PCs were attacked by a group of bandits with just below adverage stats and had a tough time, surviving mostly thanks to an NPC (being played by an extra player sitting in on the game) with well below adverage stats (to the extent that they were all joking that they should try and talk the NPC into joining them on their 'quest'). I nthe second game they fought a gang of mutants (stats originally were at adverage, but I dropped them all by 5% in liue of how game 1 went) and they slaughtered their way through them, including gunning down a big gribbly before it got anywhere near them.
Have yet to do another session/write a new scenario due to uni work/exams.