I have 2 windows which have 'dropped' and are difficult to open. Consequently one handle has snapped off, and another has weakened. Would it be simple enough to repair? Or replace window panel?

I am guessing that these windows have 'friction hinges' If so, they are quick easy and reasonably cheap to replace. The handle is also easy as long as you can get it open to get to the fixing screws!

Most windows are repairable, however depending on the aged and type you may be spending good money after bad! I would advise you have them looked at by some one that is in the trade.

You will need to check that the mechanism on the sash itself are working properly and that all mushroom/rollers all meet the keeps on the frames when closed. If the window is difficult to open and shut either the mushrooms ors are probably out of line, or the handle mechanism itself could be broken, some of these mechanisms are welded and if put under stress when shutting i.e tight.. can break after a period of time. The hinge itself could be broken but not as likely, all the above could be fixed fairly easily and should take no longer than and hour worst scenario would be £150 in total to fix, compare that to two windows needing to be replace could be upto £400-£500.