Had to redo this reply as Mat and Eddie have just said a lot of it and I would agree with them, although I don't fish stillwaters in Scotland - much to my regret and to be rectified asap
I would reemphasise what Mat said about buzzer hatches in winter. Last winter I took a fair few fish on a B&P spider, size 18 fished just under the surface film - even when ice was forming at the edges and even when there was no obvious hatch. So don't discount the possibility of fish being high in the water, even in midwinter.
As for 10lb fluoro, I think it is a bit thick and heavy for small buzzers. Your problem with breakages using 4lb, may be down to using a heavy rod and line with a 4lb leader, or with the knots you use. Not everybody agrees but I think that there are some knots that fluoro doesn't like eg the half blood knot.
Also don't persist with a fluoro leader if you can see damage on it.