Ok , this is a hopeless anoraky question but something I suddenly
wondered
last night... How come most bus routes have a simple number (eg 13,
221) but
some have a letter in them , eg W6. Why do some routes have letters
too? Its
not like they've used up all the numbers in the universe so do these
letters
denote something special (ok , I know what the N in the night bus
routes mean ,
I'm talking about daytime routes)?

I defer to those who know what they are talking about, but I believe
that the routes with letters are those that were introduced with flat
fares at a time when that wasn't the norm. For example the Wally 8
was a flat fare replacement for the 128. So presumably for most of
the routes it is just a historical thing and as there has been no need
to change the route numbers they have been left well alone.