What Mike says;
However, if we do have the time to give advisories as we call it here in the US. We have the duty to give what we call traffic alerts if we feel that targets will merge and try to provide separation (targets not touching). It's one of those things that we are supposed to provide a safety service if we can. If you see two VFR's who you are not talking to and they are going to merge, <shrug> Oh well, be careful out there. It doesn't matter what class of airspace that they are in if you are giving advisory services. Now that said, it is still ALWAYS the pilots responsibility to see and avoid (IFR or VFR)...
regards
Scott