The RM exhaust system is fairly straightforward and
the commonest sections to fail are the flexible pipes.
Here is a diagram of the standard AEC exhaust from an
old Truckmaster catalogue.

The
original AEC part number is shown with the Truckmaster
and Wasey equivalents. None of these three suppliers
is still in business but a good commercial vehicle exhaust supplier should be
able to cross reference from the old Truckmaster part
number given above. Commercial
Vehicle Exhausts Ltd used to supply some of the sections but they have now been
taken over by Dinex Exhausts Ltd http://www.dinex.co.uk/index.asp?pID={C1E258D7-288F-4819-B636-1AF5C1A5C0B2}
The Dinex part number for the silencer is 16304 and I managed to get one (July
2006) from
PETERBOROUGH EXHAUST SERVICES
11 RIVER LANE
PETERBOROUGH
CAMBRIDGESHIRE
PE3 6AH
01733 346 750
01733 346 750 (fax)
It cost just over £100 to which must be added the next day delivery charge and
VAT.
The silencer is fairy easy to fit. Make sure you
install it with the arrow on the casing facing towards the tail pipe!
Update (May 2008)
I’ve just bought some new exhaust sections. Dinex still seem to have good stock levels of them
The tailpipe (TAE22M) above is Dinex part number 16606
(£60) and the section next to it (TAE31B) is 16608 (£47.50).