The 1.4ACT is a 1.4 litre 150PS unit with cylinder deactivation (which, in theory at any rate, improves economy). If my memory serves me correctly it’s supposed to do about 60mpg on the combined cycle and emit 109g/co2. The 1.8 litre puts out 180PS but doesn’t appear to be vastly quicker in the real world (I think the 0-60 is about 0.6 second faster) and suffers significantly worse fuel economy. I had the same dilemma when I choose my 1.4 (an earlier version with ‘only’ 140PS and no cylinder deactivation) but decided that the marginally better performance of the 1.8 wasn’t worth it for the increased running costs. But that might just be because I’m getting old and sensible…