For what it is worth – speculation not spoilers and just t see how much (if any) I’ve got right before the big show.
Sam is in a coma. He is not a time traveller and is not mad. His mind is imagining he is in 1973 because that is when his father went missing. Gene, Chris, Ray &c are police officers from 1973 who he remembers investigating his father’s wrongdoing (and who young Sam sees at the wedding when his dad goes missing). Annie was injured by his father in 1973. He is back in 1973 because Life on Mars is what was playing at the family wedding when his father disappeared and that is what was playing when he goes into his coma. The scenario is therefore spun from his childhood memories.
He is a police officer in 2006/7 and his memories of 1973 police come from 1970s cop shows (The Sweeney &c – hence the knocking over of bins by the car &c) and he has transposed his memories about them onto the real people he remembers from 1973.
To get back Sam needs to realise he can go home and has had it in his power at any time (by killing himself in 1973?).
Following the Wizard of Oz theme which seems to underlie the series (Sam is “Dorothy” to Gene; Frank Morgan (the Wizard); Gene (the Tin Man); Ray (the Lion); Chris (the Scarecrow) – Sam having transposed childhood memories onto the real officers he remembers from 1973). Frank (a doctor in 2006/7) will promise him a way home. Frank will not manage it, and Annie (Glinda) will then give him the key to get back.
Cobblers obviously, but what do you think is going on?