Why would the head gasket blow on my Honda Accord 2.3SRi?  All
aluminium engine, well kept, 98,000 miles only, no apparent reason. 
Head and block surfaces look perfect and polished, like new.  Is it
likely to happen again if rebuilt?

Has the motor ever been run hot?
Aluminum is much more prone to warpage at high temps which could lead
to a blown head gasket.  BUT as I understand it, this is not a common
problem with Honda motors and most likely not to happen again unless
the motor is run at high temps.

Having owned the car for two years and done fifty thousand miles in
it, the only time the temperature has run above normal was in the last
few moments before having to cry for help to the AA.  Even then there
was no warning, no loss of power although I suspect this wouldn't have
lasted much longer as most of the water seems to have disapeared and
most of the oil had ended up in the cooling system. Neither the
engineer that has stripped the motor nor I can see anything to explain
this rather catastrophic failure, even the old gasket looks good.

Interesting.  After discussing with a racer buddy of mine that is
employed by a Honda dealership, This was the first example he had ever
heard of.  One thing of note, Have the head and block pressure checked
before having motor put back together!  Appearances can be deceiving. 
What may look nice and smooth could have a channel that allows
pressure to move from cylinder to cylinder (thus resulting in said
blown head gasket)  Remember, the head gasket failed for SOME reason,
through pressure movement (channel), material failure (head gasket
just gave out), or improper assembly (head bolts not torqued within
specs).

Profuse thanks shockwaveracing for the info... all useful. Have just
heard it was a cracked head.  Now what on earth would cause that after
all this time????  Guess we have to try and source another, or scrap
the car.