ProM
I am trying not to get involved in the general "T5 knocking" but yes, after three visits where I have tried to use the building to it's advantages, it would appear you do indeed have to go down then up.
To then, on some flights, be bussed out to the aircraft (or back from) is the final ignomy. I'm embarassed to have to explain to business visitors that this is London's answer to air travel in the 21st Century.