To replace the broken cord, you need to remove the window from its frame; so if one goes, you might as well replace the others at the same time.

When you buy the replacement cord, make sure it is the plaited type used for this purpose. This cord is now made of plastic and is rot proof.

Start by removing the stop beading which holds the inner sash in position. You can prise it off with an old screwdriver or chisel inserted close to the nails. If one of the cords on the inner sash is intact, cut it and lower the weight. You can now lift out the sash from the frame.

To remove the upper sash, you need to prise off the parting beading slotted into the frame.

Sever, if necessary, the two sash cords attached to the top sash, then remove the window from its frame. Make sure you are supporting the weights before you cut the cords. Once both sections of the window have been removed, pull out the nails holding the remains of the sash cords to the cord channels.

Lever out the small covers at the bottom of the sash weight channels, remove the weights, and detach any remaining cord.

Feed the new cord over the pulley and down to the bottom of the weight channel. If the pulley is not revolving freely, lubricate it with a drop of oil or scrape away any paint that may be making it stick.

Tie one end of the cord securely to the weight, and nail the other end to the sash. Use at least three flat headed clout nails, spaced apart and driven through the centre line of the cord. Do not nail right to the top of the sash, or it will not rise fully.

Once all four cords have been replaced, the window can be fitted back into its frame. Buy new sections of beading to replace any that were damaged when you prised it off.

If you prefer, it is possible to fit spring lift mechanisms on to windows originally fitted with sash cords. These mechanisms are virtually maintenance-free and if the tension on the spring loosens, it is a very simple job to undo the tube and re-tension it.

To fit one of these, you will have to extend and widen the cord channels which run part of the way down the sides of both sashes. The best tool to use for this job is an electric router, but these are relatively expensive and specialized tools.

An alternative method is to use a gouge and mallet to form a curved channel or chisel out a square channel big enough to take the mechanism tube.

You will also have to remove the sash pulleys and fill the holes left in the frame. Once this has been done, you can fit the new mechanism.