Yeah, it sounds like you're talking about the valve cover gasket.

The "cam belt" is more frequently called the "timing belt"
these days. If it's never been replaced before on your
Civic, it's way overdue. The replacement frequency is 90k
miles ( = 97k km) or 6 years, whichever comes first, for
your Civic, for non-severe, U.S. driving conditions. You
should have it replaced immediately by a good independent
import shop or a dealer. If it breaks, and at this age, it
could break at any moment, serious damage may result to your
Civic's engine.

A timing belt replacement is not a beginner's job.

While the timing belt is being replaced, it's an easy matter
for the technician to also replace the items I discuss
below.

Right, it sounds more like what is more commonly called the
"valve cover gasket," also known as the "cylinder head cover
gasket." See Item 2 for the 89 Civic 4-door LX at
http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&cat
cgry1=Civic&catcgry2=1989&catcgry3=4DR+LX&catcgry4=KA4AT&catcgry5=CYLINDER+HEAD+
COVER

This gasket only lasts around 75k miles/five years or so. If
you are a little handy, it is not hard to replace. Tips:

-- Do not overtighten the valve cover cap nuts. They require
only 7 ft-lbs. of torque. A $30 low range torque wrench from
Harbor Freight is an excellent investment. Wait until
Father's Day, and it will probably be on sale. :-)
-- Tighten the valve cover cap nuts evenly (bicycle spoke
fashion) to ensure the gasket is evenly pressured and so
seals properly.
-- Strongly consider also replacing the washers (item 13 at
the site above) beneath the valve cover cap nuts, too. They
tend to harden and start failing after around 100k miles.
-- Strongly consider also replacing the spark plug tube
gaskets (item 3) that fit into the valve cover.
-- Buy only OEM ( = genuine Honda) for all of the above. My
and others' experience is that aftermarket rubber parts do
not last nearly as long as OEM.

The grey plastic cover on the right (as one stands at the
car's front bumper, hood up, looking at the engine) is
indeed the timing belt cover. See item 3 at
http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&cat
cgry1=Civic&catcgry2=1989&catcgry3=4DR+LX&catcgry4=KA4AT&catcgry5=CAMSHAFT-TIMIN
G+BELT

But like I say, all these parts can be replaced easily
during a timing belt change.

www.autozone.com has a free online manual for your car.
Getting used to where certain subjects are located within
the online manual takes a little time. You can always ask
here if you can't find something.

BTW, do you have the owner's manual for this car? It has the
maintenance schedule in it.

The timing belt has been changed at least once in the cars life. ( I
cannot read the mileage on the sticker though.  I cannot afford to do
that at the moment, so on the weekend I bought a rocker cover gasket and
took the cover off the engine and replaced that as well as the
air-filter which probably hasn't been changed in 40,000kms.  I noticed
that there was a thick layer of congealed/carbonised oil inside the
rocker cover, is this normal/unusual/a concern?

Honda parts aren't cheap here and seeing as this car cost me about
US$300 2 years ago I am reluctant to spend too much money.  This week I
will take the car down to the local wash world and clean as much of the
oil off the bottom of the engine as possible so I can  see if it is
leaking anywhere else.

Matt