To me yield is quite an easy definition. It's the flight total revenue divided by the number of seats sold (and not the capacity, which is revenue per seat).
Profit = revenue - cost, and yield is a pure revenue metric, so there is absolutely no cost included in its calculation.
Airport taxes are not included as neutral for the airline (collect it from pax and give it to airports), but surcharges usually are. Credit card fees, bagages fees etc are excluded as there are considered as anciliaries revenue. So overall, revenue = flight revenue (ie. capacity * load factor)*yield) + anciliaries revenue
Yield management actually usually means revenue mangement (ie. to find the right balance between load factor and yield that will maximise the revenue).
Problem during crisis is that airline tends to protect their market share at the detriment of yields, which is wrong to my point of view. Volume-driven revenue management is also due to fierce price competition over the last few years, and also for low cost carriers the most important metric for various reason (heavily dependant on passengers numbers for subsidies and high margin anciliaries, the greater the load factor the lower the cost per passenger etc).
Yield strategies are more used when competition is very low, for example with airfrance on their african network (they know they will generate the highest revenue with very high yield but load factor at r40%).
Finally, your sentence "I need to get a yield of £50 a seat to make it work" doesn't mean anything. If it is £50 a seat available, then it's not yield but revenue. If it's seats sold, then it misses the load factor to be able to calculate the break even (corect sentence would be "to make it works, I need a yield of £50 for 60% LF, or of £60 for 50% LF)