If anyone finds this thread later and wonders if we ever solved it, well kind of. Whisks helped a little but only when the bottle was left to sit. So yesterday we were in ikea and saw some handheld milk frothers for £1. Figured it was worth a shot. It really was.
It still works best if the bottle is left sitting but we've had far less traumatic bottles since we started using it, even last nights late bottle was barely plagued by clogged teats. wish we'd found this before wasting money trying a different bottle out...
What we do is roll the bottle to get the powder mainly mixed. Then mix again using the frother and let it froth (sometimes we've found it doesn't majorly froth, other time its like cappuccino). If day time then leave it to sit, if night skip straight to next step. Stir frothy milk slowly with a fork until it all returns to a liquid consistency. Milk flows far better through teats and ds is now taking an extra ounce as he's not so tired battling clogged bottles.
This doesn't stop the starch thickening in the teats if baby decides to be lazy over bottle, but you just have to pinch those bits out.