You have to add a null terminator to your buffer before you convert it to a String.
You also have to make sure you don't over-fill the buffer.
String readPage() {
//read the page, and capture & return everything between '<' and '>'
stringPos = 0;
memset( &inString, 0, 32 ); //clear inString memory
while(true){
if (client.available()) {
char c = client.read();
switch (c) {
case '<':
stringPos = 0;
startRead = true; //Ready to start reading the part
break;
case '>':
if (startRead) {
inString[stringPos++] = '\0'; // Add null terminator
client.stop();
client.flush();
Serial.println("disconnecting.");
return inString;
}
default:
if (startRead && stringPos < (sizeof instring - 1)) {
inString[stringPos++] = c;
}
}
}
}
}