All I would add to the previous recommendations is that by far the
best way to catch the train to Salisbury is to take the Rail-Air coach
link from Heathrow to Woking, and pick up the Salisbury train there.
You can even buy a monthly return ticket for the entire trip before
leaving home.  There are trains every 30 minutes, and the journey is
one hour.  See South West Trains' web site (note that the times may
change in June):

  http://www.swtrains.co.uk/transportlinks/woking.htm
  http://www.swt-downloads.co.uk/pdfs/new2005/ITT20s05.pdf

While you're in Salisbury, stop for a day to have a look 'round --
it's one of the country's finest places to visit.  The Cathedral spire
is the tallest in Europe and lovers of old machinery should search out
the Cathedral clock, built in 1386 and still working!

When it comes to the fair itself, one of the highlights is after dark
when it's all lit up -- the row of showmens' engines in their full glory
is a wonderful sight.  Having to catch the bus home before dark is a
shame.  I rarely manage to leave much before midnight.

Julian Dyer