To reply to Hawk37: My description was somewhat condensed. We did enter a hold, hence the rapid speed reduction. Its surprising how long a hold takes above FL350. During this time we had a continuous refusal of onward clearance and no luck (at 0200Z) in getting help from the company. Limited fuel reserves precluded protracted holding and the return along track required a descent to an appropriate quandrantal level. The CIS controller gave the return clearance, but not a level change- the airway was extremely busy and there was following traffic at our level.
In my experience, most major airfields will only pass an SID in the initial clearance, which implies activation of the filed flightplan. This flight plan and stored company overflight clearances take care of the en-route. At some international boundaries a readback and verification of the overfilight clearance is required ( usually for military flights, airlines operating ad-hoc services, or temporary flights by charter airlines) It turned out that our entry was refused because the controller was looking at the previous day's clearance number.
My questions still remain.......what are the continuing/re routing options?
It occurs to me that, in the event of total radio failure, we would be expected to follow our flightplan to destination anyway! Could we then expect a fighter escort if we penetrate one of these international boundaries without passing a clearance number or even talking to the control centre at all?