The spacer tube will need the inside edges filing to remove burrs and any sharp edge. There might also be a welding ridge on the inside that will need to be filed away.
Bashing the spacer tube onto the headers could well cause damage and stainless can gall (self weld) making it almost impossible to move (on or off). The best option would be to have the new pipe slightly swaged to give a good fit without being too tight.
Xtreme Stainless (near Stafford) use silicone sealer on pipe joints. It lubricates the fitting and causes no problems even on very high heat areas.
Stafford is not much help for a the York area. Google found these. Give them a call.
http://www.cskfabrication.co.uk/contact/4589654394