We usually use Crovect, it is good for treatment, although purchased Dysect last time as it was a bit cheaper and provides slightly longer protection. We kept some Crovect back in case it was needed for treatment though as have had sheep with severe reactions to Dysect when used for treatment.
I am surprised that Crovect wasn't effective onnyview, as it has always been really good at treating, just spraying a bit on the affected area without any need for clipping it always seems to have done the job, fortunately have seen any flystrike for a couple of years or so now. Obviously with 16 lambs affected I wouldn't want to be using it again though, just seems odd.
We usually use a herbal spray too at the beginning and the start of the season to limit the use of chemical sprays, it also gets used on any meat lambs that we don't want to have a withdrawal restriction on, as well as being the only spray used on the show lambs through the summer as I react to Crovect etc.
We stopped treating the adult sheep altogether a few years ago, once they are shorn they don't seem to suffer any fly strike and we haven't had any cases since doing so. However have since been advised that it would help keep flies away from the ewes udder and hence help prevent mastitis if we used it, not sure about this, but guess it would help limit the fly population a bit.
re Click, the advantage of this is that it should only need to be applied once through the season, whereas Crovect may need 2 or 3 applications. However as said earlier you cannot use Click as a treatment, it works by inihibiting growth of the blowfly larvae so will have no effect on larvae that have reached the stage to cause strike damage.