We typically use Yield to refer to revenue per passenger. Other airlines use it to refer to revenue per passenger km, or passenger mile.
In terms of revenue per passenger, it's indeed related to the average fare. "Gross yield" is the average all-in price, so if the average one-way fare is £150 plus £30 taxes and charges (ie the average passenger has paid £180 for her one-way ticket), then the gross yield is £180. "Net yield" is what's left after taking out taxes and charges (ie what comes into the airline's coffers) - £150 in this example. And some carriers (BA if I recall correctly) also refer to "net net" yield, which is the net yield with commissions subtracted.
If a carrier says "we need a yield of £50 to make it work", I'd hazard a guess that they are talking about net (or maybe net net - but probably not gross). However the statement is meaningless unless they've also specified what load factor they're talking about.
C.