KelvinD - T2 has a segregated domestic connections route so no, an international connecting passenger does not have to leave the 'airside' environment to reach their flight from T5. The only way to achieve a smoke would be pass through arrivals and immigration at T5, have a smoke in one of the landside smoking zones located between arrivals and the MSCP then connect to T2 landside on the Heathrow Express (free), have another smoke outside T2 departures then proceed to the gate through security etc. Bit of a pain for the sake of a smoke but could be done if you have the time wiseguy.