You’re right – superstores that supply standard-size windows are bound to offer lower prices than suppliers that only fit bespoke windows. Presumably, though, an old cottage is likely to need windows in non-standard sizes, so the off-the-shelf option will not work for your project. Speak to a few window fabricators who use one of the well-known PVC-U profiles and get them to price a similar range of windows. Measure the rough window sizes you need and sketch the external view of each, showing clearly where openers are to go. Ensure you include:
● Removal/disposal of existing frames
● Installation of the new windows
● Provision of internal and external trims and sealants to make good the walls
● Allow for all access towers/scaffolds as necessary to safely complete the works.
This will give you a basis to compare costs and any product pros and cons.