The validity of section 21 notices (or technically, when one can be served) became simultaneously more complicated and less so after the Deregulation Act 2015 took effect, together with changes in deposit regulations. There is a lot to think through and of course it all varies by date of tenancy.
On the basis that a lot of tenants, a lot of landlords and letting agents and quite a few advisors haven't really got to grips with current requirements for a s.21 notice to be valid, I thought a flow chart could help, assuming it was possible.
It turned out to be sort of possible- ish, as least as far as statutory validity goes. So here it is - PDF download
[Update 03/06/2016 - flowchart now updated to include HMO licensing requirements. Update 05/12/2016 - tidier version and small error correction]
The usual disclaimers apply - it should not take the place of proper legal advice, or even be relied upon - guidance only.
There are caveats - lots of caveats.
It doesn't cover every variation or eventuality (for instance, all the deposit stuff assumes no change in landlord over the course of the tenancy, and s.21(4)(a) notices are an additional footnote).
The usual rules on service, named tenants etc. aren't dealt with at all. Nor (for space reasons) does it mention that the Govt 'How to rent' booklet must be the most recent edition at the time the current tenancy started (and that is a tricky one to check but start here.)
The Ravenseft Properties Ltd v Hall  EWCA Civ 2034 precedent on prescribed forms and notices 'substantially to the same effect' will probably apply to the post 1 October 2015 prescribed s.21 Notice (Form 6A) but given the necessary detail in the notes, I find it hard to believe that not using the Form 6A would not amount to defective notice, unless a very minor error or difference.
And because I couldn't fit it in a box, there is no mention of the fact that for a post 1 October 2015 tenancy, it is impossible to serve a s.21 that will expire at the end of a six month fixed term unless it is handed to the tenant in person during the day time on the very next day after the first four months of the tenancy, assuming that isn't a weekend or bank holiday (otherwise, the usual rules of service mean that the notice must expire a day or two after the end of the six month fixed term at the earliest).
I hope it is of use to all involved in PRS tenancies in England. I'll add a page for it in resources section of the site. I may even update it come the next set of changes, or October 2018, whichever is earlier.