It is worth noting that the sleeper service appears to take much longer to complete on the 777 rather than the 747.

When doing the BAH-LHR run on the 747 I would normally get my cookies 10 minutes or so after take-off. But when the equipment changed to a 777 I tried asking for the cookies on two separate and had to wait half an hour both times.

This seemed to be because the crew had more peoples' orders to take, prepare and deliver. As the BAH-LHR flight only allows six hours sleep I now do not bother ordering the cookies and proceed straight to sleep.