In tidying up some old libraries, I seem to have inadvertently removed ESP8266WiFi.h and its associated files.
I have tried uninstalling and re-installing IDE, also removed and re-added the 8266 Boards.
How can I get these files downloaded and put back?
The ESP8266WiFi library is bundled with the ESP8266 core for Arduino boards platform. When you install the platform, you install the library too. The libraries bundled with platforms are only accessible when you have one of the boards of that platform selected from the Arduino IDE's Tools > Board menu. So likely the cause of your problem is that you have a non-ESP8266 board selected.
Indeed, that is the cause of my problem.
I had been working on a different board and was just compiling without uploading, therefore hadn’t changed board type!
Many thanks for the fast response
You're welcome. I'm glad to hear it's working now. Enjoy!