In reply to Simon Caldwell:
Heatwave was originally a V Diff that followed the line of least resistance up the buttress.
I'm not sure why, but the route description was amended to straighten out the first pitch, making it harder, so the grade was bumped up to Severe - but the original start is still there and quite climbable at V Diff, it isn't normal grade creep/reclassification.
The original line, for its first pitch, goes to about halfway up the upper of the two grooves; you then have to step down (a bit awkwardly) to the groove below and finish up that (the new description has you carry on up this groove).
After that it's as per the current guidebook description.