If I had 121.5 already on the standby frequency (and if not, why not?) then I too would be very tempted to cut the middle-man and go straight to the chaps at D&D. Otherwise, I wouldn't faff around dialling in another frequency because of the real risk of getting even that simple operation wrong - I reckon you probably lose at least 100 IQ points during moments of intense stress like that.
Squawking 7700 would be a VERY good idea and will get D&D involved anyway, regardless what frequency you're talking on.
I agree that there are better units to talk to when you're crossing the Channel. For the Dover to C-G-N route I always use Manston.