Oz,
If one reads between the lines, one can duduct that ATC no longer have any CTA conflict, and don't have anything planned for the chunck of airspace one is in, or going to descend through, prior to going OCTA.
The AIP requires a report to ATC when leaving a level that has been maintained.
Most jet transport and pressurised aircraft descend more or less on a 3 times profile; eg, Fl300 into Alice, ~ 2000' AMSL, threfore 28000' to lose, therefore TOPD ~ 84nm, give or take wind, circuit, inst app allowances.
I wouldn't have thought I'd ever received such a clearance 20 minutes prior to requiring it though.