Trying again...
float value;
memcpy(&value, ab.constData(), sizeof(float));
qDebug() << value;
QByteArray ab("\x3D\x7E\xBF\x26",4);
float value;
memcpy(&value, ab.constData(), sizeof(float));
qDebug() << value;
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I get 1.32875e-15
On the website, it converts to 0.062194012105464935
HOWEVER... when I reverse it all as input to the website, i.e. 26BF7E3D, it returns 1.32875e-15
This also works for negatives!
So based on the way I was doing it (incorrectly) worked for positives, broke for negatives. I will have to reverse my positioning in my QByteArray and then the new method will work.
Thank you.
A very useful lesson 
Shaun
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