Hi Anthony, I find alot of these old sliding sashes have been painted shut and have become trapped in the "track". Persistence with gentle leverage under the window sash, running a stanley knife down the side between the sash and the track will usually work. If it is still jammed you can remove the front bead of the track (it will just be pinned with panel pins) by gentley tapping a chisel in between where the bead meets the frame, go steady though so you dont break the bead. Once you have removed the bead on one side then you will be able to free up the sash at that side, repin that side back on mabye if it needs it pin it on a little further outwards giving it a little extra room for manouvre, then repeat it at the other side, this will free up the sash if it is jammed in between the beads.
Good luck
Yorkshire Chippie
July 2008