Some useful replies above.
IME, the few times I have asked for a Traffic service, I have been told that only Basic Service is available due Controller Workload. In fact, I don't think I've managed to obtain a Traffic Service yet!
I have had mixed experiences with a Basic Service. Sometimes I get passed lots of useful traffic information, but on the one day I really needed it (sunny Sunday last autumn, when the skies were HEAVING) I got no traffic information whatsoever.
We passed quite close to others on multiple occasions that day.
Monkeyscribbler, good luck this weekend...
