To answer my own question, there appears to be a bug in Hubitat but this thread solved the issue:
Setting String variable from HTTP GET gives java.io.StringReader@56e720 - 
Rule Machine® - Hubitat
The extract variable is indeed now showing a value of 6.
Thanks for the tutorial. My plan is now to create a number of rules by using IFTTT to populate a spreadsheet on a daily basis.
