Could somebody explain how they get the last 16 teams?
There are 24 teams, 12 of which will be winners or runners-up in their group.
The other four teams (if I understand it correctly) will be third place teams

According to my wallchart, they are:
third from groups BEF
third from groups ACD
third from groups ABF
third from groups CDE

But what happens if, for example, group F provides third place for two of the above?
How do they decide who is third in six groups to get four teams?
or have I missed something?

Think your wall chart is just confusing matters. I was under the impression it was simply the top four third-place finishes.

But there are six groups, so there will be 6 x 3rd place finishers, how do they select four from those six?

The four with the best records after the group stage finishes will progress. The other two go home.

Ive got two wallcharts, they are both in agreement

http://www.dailymail.co.uk/sport/eur...ls-France.html

If you look at match 2 in the last 16, it says : winner of group D vs Third group BEF

I'm trying to put it on a spreadsheet, so all I need to do is enter the scores. I never had this trouble with the world cup

So which one of these four will be playing against the winner of group D on the 25th June ?

You won't know that for definite until the group stage reaches its climax.

How can their be two third place teams in group f? It goes through points, GD etc all the way down to co-efficient so there can be no ties in one group.

How do they decide?

I think what the poster is asking is:

when it says 'third B/E/F', if you look at the records of the three countries in those groups and the team in F has the best, they go into that slot.

But when you get to 'third A/B/F', if the team in the F has the best record of the third placed teams in those groups, they obviously can't go into that slot as well.

The actual combination depends on which groups have a third place qualifier going forward, but Wikipedia has covered this question in more detail.

Well as only four go through, there's likely to only be one or two combinations that works. So if A, B, C, and D happen to be the best four, the first one (BEF) HAS to be the Group B team. Then the third (ABF) can only be A, and so on. C and D can then go into either, but it's pre-determined what will happen already.

The wiki page has a nice chart which lists what will happen in each combination, if it helps:

https://en.wikipedia.org/wiki/UEFA_E...Knockout_phase

The Group D winner plays the best third place team from B, E or F. If that is F, then the Group C winner plays the third team from A or B.

Its dependent on which 4 Groups the qualifiers come from

I was trying to say that the third place in group F could be good enough to get through on two of the round16 games
Match 2 = winner group D v 3rd group BEF
Match 5 = winner group C v 3rd group ABF

so if 3rd in group F is better than 3rd in groups A,B and E, they can't play in match two AND five, so there must be some other criterion...

See here. Clear as can be!

The wallcharts simplify the qualification a llittle. There are a couple of permeatations of finishes where it would come down to a draw as to what 3rd place team goes into which group but that has already been done.

Every outcome already has an allocation of where each team goes eg if the four are from A, C, D and F then the rules already confirm where each team goes.

OK lets make up an example

The best 3rd place teams are:
group F - Portugal
group C - Germany
group D - Spain
group E - Italy

Who plays the Winner of group D on 25th
Who plays the winner of group B on 25th
who plays the winner of group C on 26th
who plays the winner of group A on 26th

Have a little go at this - might help?

http://sky.thesportspredictor.com/fo...uros/wallchart

When you look at that Portugal would be better off finishing runner up than winning their group.

Was there any criteria on the two group winners which get to play runners up?

Group D Winner v E3
Group B v D3
Group C v F3
Group A v C3

Use the link that Carnoch has given you in post #15 and scroll down to Knockout Phase and it explains how it will work.

As above.

It wasn't very clear until I saw the tables under the heading "knockout phase"

Thank you for your assistance

Have a Play with this Simulator to see the possible combinations...

http://ultra.zone/UEFA-EURO-2016-Group-Stage

Yep, as much as a little concentration may be needed, it is very obvious. Although it does make a 24 team tournament look as ridiculous(because it is!). 36 of the 51 meaningful games just to lose 8 of the 24 teams. Almost guarantees every big side into Last 16.

However it then makes the knockout phase of the comp too long for me. Due to the odd slip up in the group its likely we'll be seeing say a Germany vs Italy in last 16, and then some of these one-off game seeing the lesser team that gets battered cling on and get through on pens. Obviously this is as all tournaments, but id prefer knockout rounds for 8 teams max.

Anyone remember 24 team World Cup finals with this exact same format? Was it 1990 and prior?