Shannan v Viavi Solutions UK Ltd  EWHC 1530 (Ch) is a recent decision covering a number of points on powers generally and on powers of amendment in particular. Amongst them:
No particular formality is required for the exercise of a power (statute apart) unless the trust instrument expressly or impliedly so requires; and the test for implying a requirement is the tough one of necessity .
Retrospective amendment is authorised, where the power of amendment is in suitable terms; remedying a previously invalid exercise of a power is permissible but altering accrued rights is not (though surely an express power to do so would be effective), nor is nullifying what was effective when done or validating a breach of trust (-).
There is a discussion of the ability to sever the valid from the invalid parts of an exercise of the power (-).
Exercising a power incidentally, by doing something which can work only if the power is exercised or by reciting a previous invalid exercise, is also discussed, referring to Davis v Richards and Wallington Industries Ltd in the first case and Re Shinorvic in Jersey in the second ( et seq. and  et seq.).
Nicholas Le Poidevin, Q.C.
New Square Chambers