Within the experience of my career I have covered this problem many times,
all according which sash cord has to be replaced, if it’s a front one (inner frame) take off the outer wood stays then remove the frame, and make sure you tie off the cords so the weights don't drop down inside the channel.
Once frame is out remove old cord through the top runner hole, if sash is broken the remainder will be on the weight.
Before actually pulling cord out on whichever end is easier to locate tie a smaller string/cord to this so when replacing sash cord you relocate it to it’s correct position.
With the side blocks removed locate position of the other weight then measure from bottom of weight to pulley.
When refitting new sash cord make sure weights are level before tying them off.
Refit all panelling and beads.
For outer frame repeat process and also remove one separation bead to release outer frame.

Hi needs to be replaced completely

I would say advertise in the wooden window section on here.
愿一切都好！

Here is what is available in the sash window market

Full replacement windows

A full replacement product for only one window is a sizeable investment with an average of £1800+vat+ fitting. (per window) In 12 years of sash windows i have only had to advice 2 customers that a full replacement is required.

最大值为钱

99% of our business is to replace the two sliding sashes with a new pair of timber double glazed sashes to match your existing.

This is an industry standard practice and something that we compete on price and quote match all the time. That said our windows are often warmer, certainly made from the most durable timber and best paint system available, and will always be better draught proofed and slide more smoothly thanks to our design.

They will slide within the original box sash frame which we can renovate or insert our premium draught proofing and renovation design, leaving you with a perfect finish and the smoothest sliding action, whilst retaining all of the traditional integrity of the original box sash frame.

By renovating in this way you will realize maximum value for money, less dust and no damage to plasterwork or the need for wall painting and decoration. Your blinds and soft furnishings will fit exactly as before and you will be saving the period and heritage of your home.

We provide a guarentee to be within £100's of these estimates:

1 Double glazing upgrade. £1250 incl vat and fitting: New, timber sliding sashes, draught-proofed within existing box sash frame with no paintwork:

2 Premium draught-proofing installation & renovation with double glazing upgrade. £1500 incl vat and fitting.

Complete protection to your external frame incl new rot resistant sill & hardwood timbers to protect your original box sash frame like new.

New premium metal wheel pulleys for the smoothest of sliding actions,

New timber sliding sashes as above within the finished existing box sash frame.

All works Conservation approved.

The tradition of the original with the smooth sliding action and air tightness and warmth of a new window

Huge saving on full replacement windows

Short answer is carpenters and joiners. I’m a carpenter by trade however im a sash and casement window specialist. So I only work with timber windows and have done so for over a decade.
Many thanks
Luke

The question was “who covers this type of work”. The answer is carpenters and joiners.