In 2015 I commissioned a carpenter to supply and fit new window frames made out of hardwood with a like-for-like appearance. The original leaded light panels were reinserted.
Be wary of double-glazing companies offering wooden frames with double-glazed units. I admit these are nicer than PVC but unlikely to be made to match the look of 1920s/30s windows. This is partly because more space is needed for the lock system, double glazed units and made for mass production probably without genuine appreciation for period restoration.
My windows just have 4mm glass with simple screw in locks and secondary glazing fitted on the inside. I do not need a FENSA certificate.
In 2017 the same was done for my side windows – see blog.
The apex facia was also replaced in 2015. I painted this and the windows. The objective was to highlight the building’s 1920s architectural features.