I have ended up creating my char array one value at a time but now I have a new problem.
When I send the data to my server, there is a /r appended to the end of the string (looking at it in Node-red).
My last few lines of code are;
char rew[12];
sprintf(rew,",10:%1.1f",therm.value);
strcat(foo,rew);
Previously when I done it all in a single sprintf line, there was no /r at the end.
Any ideas?