"The true rule in my opinion is, that if there is
any agreement between the principals with reference
to the contract guaranteed, the surety ought to be
consulted, and that if he has not consented to the
alteration, although in cases where it is without
inquiry evident that the alteration is
unsubstantial, or that it cannot be otherwise than
beneficial to the surety, the surety may not be
discharged; yet, that if it is not self-evident
that the alteration is unsubstantial, or one which
cannot be prejudicial to the surety, the Court,
will not, in an action against the surety, go into
an inquiry as to the effect of the alteration, or
allow the question, whether the surety is
discharged or not, to be determined by the finding
of a jury as to the materiality of the alteration
or on the question whether it is to the prejudice
of the surety, but will hold that in such a case
the surety himself must be the sole judge whether
or not he will consent to remain liable
notwithstanding the alteration, and that if he has
not so consented he will be discharged. This is in
accordance with what is stated to be the law by
Amphlett, L.J., in the Croydon Gas Company v.
Dickinson ((1876) 2 C.P.D. 46, at p.51)."