I think that maybe a walk to the Bath Road bus stop may have taken almost as long as a walk though the tunnel? Is that permitted??
It therefore appears that you could park in any suitable car park and take a bus to any terminal from any airport bus stop - the OP won't be the only person who has used Heathrow and departed from one terminal but returned to another. I've done that myself both intentionally and unintentionally (when the BA computer fiasco struck and I was bumped onto another airline that did not return to T5 where my car was parked).
Glad it all worked out in the end for the OP. I shall be doing battle with T5 again within the week.....