Hi Stuart,
I'm going to stick my neck out and say that the majority of those people on a single shot of Lantus tend to take it before bed and at the same time each day. Other people on a single dose find it better first thing in the morning. There's not really a wrong time of day so long as you keep the injection at around the same time each day.
When I used Lantus on a split dose, I used to inject twice daily at around 8am and again at 8pm.