PaulS:
if you want to use the Arduino version of the String class, you should have your head examined, and, if you persist in the fantasy that the Arduino has oodles of memory, look at String() - Arduino Reference
There, I fixed that for you.
I beg to differ. This advice was OK when most users were using an Arduino say of the AVR class with 1 or 2 k of RAM. Now the Arduino IDE supports devices with around 80k of RAM like ESP or ARM class processors. This will only improve.
Anyway, the Arduino core web server classes for the ESP8266 use the String class, so you are stuck with it even if you don't like it.
I know that some people have difficulty accepting that all their old knowledge is now of no use. All that desperate fiddling about with character arrays to try to extract a few bytes of data etc. I've seen this phenomenon in industry when I still had to work with people clinging to their old technologies like Cobol, IBM mainframes, Lotus Notes etc. etc. instead of accepting that times have moved on and learning something new.
Any, lets face it, the Arduino environment was designed to support even people who just want to light a few leds and basic things like that and why should they be forced to use tortuous methods to achieve simple things ?