<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Unterwegs:
Another option is the Underground, but you have to change trains from T3 to T4 (non stop from T4 to T3). Likely the least convenient and longest.
Walking is not an option. It is far.</font>
If you take the Underground you'll need to change @ Hatton Cross (1 stop) and cross to the other side of the same platform - I do this all the time & very convenient.
As for walking - it's miles/kms away and impossible to do - you need to leave the main part of the airport through the car tunnel, go around the parimeter fence, through the BA maintenance area, etc...
I'd take the underground.