This s what the rules say, as posted by alanw on Bashley's forum...
Regulation 6.5
If two or more Clubs (“the Merging Clubs”) are proposing a transaction or series of transactions that result in the merging or consolidation (“the Proposed Merger”) of those Clubs into one Club (“the Merged Club”) then a formal application to do so must be received by the LC and the league(s) of which the Merging Clubs are members by 31 December. A Deed of Agreement, which shall be legally binding on all parties must be submitted to The Association by 31st March in the year immediately following receipt of the application. The LC shall determine at its absolute discretion where the Merged Club is to be included in the NLS for the following season subject to the provisions of item (v) below.
In arriving at its decision the LC may apply the following minimum criteria:
(i) The requirements of Rule 2.9 Transfer of Membership of FA Standardised Membership Rules for the current season must be met by each of the Merging Clubs. If one or more of the Merging Clubs is subject to an insolvency event then Rule 2.9.2 shall be applied to such club(s), otherwise Rule 2.9.1 shall be applied;
(ii) the proposed playing name of the Merged Club must be acceptable to the LC; and
(iii) the Merged Club must have security of tenure to a ground that meets the relevant
ground grading requirements;
(iv) Any other criteria that the LC may from time to time deem to be appropriate;
(v) The Merged Club will ordinarily be placed at the lower of the Steps at which the Merging Clubs ended the Playing Season in which the application is made. For the purposes of this Regulation, if one of the Merging Clubs has finished that Playing Season in a relegation place, then they will be deemed to have ended the Playing Season at the Step to which they would have been relegated without the Proposed Merger proceeding.
Any decision regarding whether a proposed transaction or series of transactions falls to be
considered under this Regulation shall be determined by the LC at its absolute discretion.
Of course, that very last bit gives the FA a getout clause to decide whatever it wants. Bashley could claim that NM have folded and they are merely taking advantage of a vacant ground to move to somewhere that gives them better possibilities for future progress. ie. not a merger at all.
The examples quoted of other mergers are all correct, but most date from a good few years ago. I have it in my mind that the FA tightened this up much more recently to stop clubs getting an artificial "bunk up" when they were finding promotion too difficult on the pitch. Hinckley is a perfect example of this. Athletic had the crowds, the history, the better ground (facilities and location) and the potential. All Town brought to the merger was their higher status.