It's definitely, or at least mainly, lockdown. At the start my builder ordered loads of building materials - bricks, mortar, plaster, etc, etc in advance anticipating supply issues and they are all in my garden and driveway just waiting to be used. I don't think he anticipated quite the magnitude of delay with the doors and windows that we've had, so this aspect of planning could have been better with hindsight.
The main issue was that there were some problems with the steel designs that we had to rectify in December which lost us a couple of weeks, and as soon as these were righted, the snow set in and for the last four weeks now we've had either snow, rain or ice most days.
Builder has been fab throughout to be fair.