Combinex contains Fasinex plus wormers. If you only want to fluke then get Fasinex - it's cheaper and you aren't 'wasting' wormer. I appreciate that getting this, that and the other isn't so easy for small numbers - but would your vet dispense a smaller amount for you? Fasinex has a long shelf life and comes in 0.8L packs as well as 2.2L and 5L. It's 1ml per 10kg bodyweight, so if I have my arithmetic right, 800ml would treat 15 x 60kg sheep 8 times. If you dose 3 times over the cooler / wetter months, so long as it has at least 2 years left to run when you buy it, you won't be wasting much if any.
We have been using Combinex this year as, with our current weather, the fluke persists all the (cool, wet so-called) summer, and the worms survive all the (mild, wet) winter...
- but in Somerset I guess these parasites are all still seasonal as they should be. So if I were you, colliewoman, I'd use Fasinex over the winter and whatever wormer in the summer as needed.
And with your numbers and history I wouldn't get too het up about rotation of drugs, either, I'd just dose at the recommended intervals and, most importantly, at the recommended dose (having checked weights to be sure of getting the dose right) and with the correct technique.
With wormers they are now saying that if resistance is going to develop it develops whether you rotate or not, and there is now a school of thought that it may be best to use one wormer until resistance has developed to that, then switch to another wormer until resistance develops, and so on.
Personally I suspect resistance is much more likely to develop where dosing is less accurate (underdosing due to underestimating weight and/or failing to calibrate and test dosing gun every time), too infrequent or delivered badly (bad technique resulting in underdosing.)