[Exim] Sudden increase in queue length & no. processes

Our Exim queue has jumped from a usual length of 1 or 2,

to approx 100, consistently.  This has happened in the last day or so.
(Also, the no. of Exim processes jumped to over 200.
and so we changed exim.conf:
    queue_only
    queue_run_max = 40
for now, at least.)

* Why would the queue suddenly expand?

* How do you go about working out why a message spends so long in the
queue? (Even a message from me to another internal user?)

    Thanks,
    Niall

Time spent on the queue: all messages
-------------------------------------

Under   1m      282  19.2%   19.2%
        5m      142   9.7%   28.8%
       15m      161  10.9%   39.8%
       30m      163  11.1%   50.8%
        1h      361  24.5%   75.4%
        3h      362  24.6%  100.0%

Time spent on the queue: messages with at least one remote delivery
-------------------------------------------------------------------

Under   1m       22  40.0%   40.0%
        5m        3   5.5%   45.5%
       15m        1   1.8%   47.3%
       30m        6  10.9%   58.2%
        1h       14  25.5%   83.6%
        3h        9  16.4%  100.0%