Now, pay attention, this is a bit confusing. The Official Charts Company have changed the chart rules so that a CD single containing a previous hit single as a b-side (well, a remix or different version of it) while that original hit single is still available as an a-side will be chart eligible. Previously it wouldn't have been.
Why the change? Well, this is where it gets a little bit complicated. Previously a record label who wanted to include a remix or rework of a former hit single as a b-side on a new single could sidestep the rule that disallowed two different but concurrent physical CD single releases carrying the same track by deleting the original CD single release from its catalogue. Doing so would disqualify the original single from chart eligibility, and allow the new release to count. In the olden days the fact the original track was also available via download stores didn't matter, because the digital sales of the original track didn't count for the chart unless there was a physical release in circulation, so by deleting the CD they were also stopping digital sales of the original track from counting, even though it was still available for download. That, of course, has now changed, because download sales now count even if there is no physical CD on sale, meaning that if a label wanted to release a new CD single featuring a new version of a former hit single they'd have to not only delete the former CD single but take the original track off all the download platforms too, which they would never want to do. You're all following this, right?
Anyway, it is with that new quandary in mind that the rule has been changed. I've no idea how many CD single releases this actually affects - probably two - but presumably you'll all sleep a bit better tonight knowing that labels who put a former hit as a b-side on a single release are no longer jeopardising their chart eligibility. I know I will.