So far I've found that ATCOs make sure I've received and read back any vital instructions or info - e.g. "Golf Delta Foxtrot, I say again traffic crossing ahead of you right to left height unknown". "Golf Delta Foxtrot has traffic in sight thanks".
Or more recently : "Golf Delta Foxtrot, you are aware that the minimum altitude above xxxx is two thousand four hundred". "Affirm, maintaining two thousand six hundred on the QNH, Golf Delta Foxtrot". This was on the first day we could fly after the WTC...
I'm glad that someone else is also keeping an eye out for other traffic although I do realise it's primarily my responsibilty. (So many thanks to the ATCOs at Farnborough, Brize and Boscombe)
---Mik