I've mentioned this a couple of times before but I thought I might as well quote this bit of genius from Jim Smith directly;

"Okay. Obviously the first three and last two are bolted in place.

Okay, here's the criteria I'm using.

1) 'Herne's Son' takes place in 1209 according to Anthony Horowitz's novelisation. This ties in with King John's real life (abortive) invasion of Wales through the Marches. I wouldn't feel bound by this, except that -
2) In 'The Time of the Wolf' the Sheriff says it's "The year of our Lord twelve hundred and eleven", which ties in with -
3) Hubert De Gisgard's comment in 'The Sheriff of Nottingham' that the outlaws (assuming under Huntingdon because of the interregnum) have been in Sherwood for "Two years".

So, that gives us a frame to work in. The rest is dictated by the weather (Yes, I know it was shot in one year and over nine months or so, but as shooting order is impossible from a narrative pov, we're not bound by it) and a few other factors, including the dynamic in the band. Real history can also come to the rescue at some points.

1) In 'The Cross of St Ciricus' Marion tells Robin that "If I loved you in the way you want me to, then you'd know and there'd be no need for words", while John responds to an order with "I know when the Earl's son puts me in my place", both of which demand this comes nearer the beginning than the end of this run. (Visibly shot in Spring.)
2) In 'Crom Cruac' Marion realises when she's talking to the Abbot that she loves Robin (Visibly shot late Spring/Early Summer.)
3) The Sheriff tries to hang Much in 'Adam Bell' (Winter) an event referred to in the past tense in 'The Sheriff of Nottingham' (High Summer).
4) In 'Rutterkin' (Summer) King John says he will deal with "de Rainault, when he returns", which makes most sense if positioned immediately before 'Sheriff'. The plot concerns an assassination attempt on King John in which David, Earl Huntingdon is ostensibly involved. There was a real one of these in 1211. I do not believe this is a coincidence. This is the kind of thing Richard Carpenter knows.
5) Robert and Marion kiss for the first time in 'The Betrayal' (visibly shot in the very depths of winter).
6) The last scene of 'Adam Bell' seems to take place months after the rest. The boy has been with them a while, there are leaves on the trees and John's beard has grown from invisible to vast. (Yes, it's a reshoot, but we're playing fiction).
7) In 'The Inheritance' (Late Spring) Marion is willing to stay with Robert at Camelot forever, if needs be. So it must come later. They are also sleeping together at the beginning of the episode.
8) The weather/fauna of 'The Pretender' are that of a wet late spring.

Herne's Son (Early Spring 1209)

The Power of Albion (Spring 1209) [Within weeks of 'Herne's Son'.]

The Cross of St Ciricus (Spring 1209) [Band still not settled down internally but Robert now famous.]

The Pretender (Spring/Summer 1209) [Robert more famous. Unfortunately King John's second marriage took place a decade earlier.]

The Betrayal (The winter of 09/10) [The King is willing to believe Marion will betray Robert, but the band are close knit when seen together. Will has a strop, mind.]

Crom Cruac (Summer 1210) [Marion realises she's in love with Robert.]

Adam Bell (The Winter into Spring of 10/11)

The Inheritance (Spring/Summer 1211)

Rutterkin (Summer 1211)

The Sheriff of Nottingham (Sheriff sacked by King, "Two years" since 'Herne's Son', High Summer.)

The Time of the Wolf (High Summer 1211)"