That's the kind of challenge I relish!
Often, in situations where there's a sudden spike in fox predation, analysing the data reveals that in that particular year the flock experienced a higher than normal proportion of low birthweight lambs.
Obviously not all farmers record that data, but to be honest if they don't measure it they can't mend it, and any attempt to do so is merely ****ing in the wind.
Low birthweights might be caused by a numbed of things. For example, poor ewe nutrition, or disease (eg, toxoplasmosis. Toxo is very common, and the majority of sheep will become infected with it at some point in their lives. Where there's a low level persistent infection in flock it'll often pass unnoticed, but when a new infection occurs in a previously naive flock the effects in that year can be devastating. If infected in early pregnancy, ewes will repeat to the ram and either lamb very late or be barren. When infection occurs in mid pregnancy, ewes will abort. But when it occurs in late pregnancy ewes will produce live lambs with low birth weights and low vigour. Perfect fox food! In subsequent years, ewes are immune, so the problem appears to have gone away, and predation drops accordingly. Vaccinating a flock against toxo is pretty much guaranteed to reduce the number of lambs predated).
You mention that it is a small flock, and they have their own set of problems. Age structure is one example: In smallholders flocks it is quite common for older ewes not to have been culled when they should have been, for "sentimental reasons", so insufficient young sheep are retained each year to replace them, and the age profile of the flock advances year on year. It is well documented that there comes a point in a ewe's life when her ability to rear a lamb and produce quality colostrum declines sharply, again resulting in potential "fox bait" lambs. When an unnaturally high proportion of the flock reaches that age category all at the same time you'll see a correspondingly high level of lamb mortality in that particular year, leading to higher than normal predation.
So, there's just a few examples of what might have caused the situation you witnessed, all of which are solved without shooting any foxes. You shot the fox after the event, so didn't prevent the 17 deaths. Being aware of, and adressing issues such as those I've outlined above (or others. These are just examples) would have done.
Saving him the cost of a call-out fee