Answering my own question now!! Thanks TBS. Just visited T5 by way of recce on way out of London to LHR hotel before tomorrows flight, chatted to a few Hoppa drivers. There is the usual confusion of people getting on the wrong bus!

The Hoppa buses depart T5 from bus stops 26 and 27 at departures level. They are renumbered for T5 so that the normal H1, H2 etc have a "5" prefix for the same route but serving T5, so H1 becomes H51 and H2 becomes H52 etc etc. The ordinary H1 H2 etc continue to serve T1/T2/T3 as normal.

The Hoppa delivers you to the departures level also, as opposed to T1, where you arrive and depart from the arrivals level.

Hope that helps anyone else looking for the info.