Hi Colin,
Thanks for that. I've just had a read at that thread where Jamie explained the volcano plot and noticed that the example in question looked a bit different to mine. It seems that poster was comparing multiple groups, so there is an F-score on the x axis in that case:
So I understand that. But in my graph, there are only two groups. My x axis looks different:
In this post here, Jamie said that for the example given with only 2 groups,
"Considering the volcano plot, it looks like there isn’t an F-statistic being run. When there are only 2 categories, just the clr mean difference is calculated (which is essentially a log fold change). If you have negative log fold change, that is indicative of decrease (since log(x) < 0 for 0 < x < 1), whereas a positive log fold change is indicative of increase (since log(x) > 0 for x > 1)."
(Sorry, can't find how to link the quote from that post!)
So from what I can gather, it seems like features appearing in the top left of the graph are just as significant/valid/important as those in the top right, in this case where there are only two groups? Is that correct?
Thanks again 
Lindsay