Also an anonymous poster has left the following interesting comment:
A rule change to the Labour Party Constitution in 94 also rendered the Deputy Leader redundant if something happened to the PM. The rule change that year stipulated that should the leader become unavailable while the party is in office, (I paraphrase), 'the Cabinet, meeting in consultation with the NEC, shall elect one of its number'. So Brown or Straw.I can only recall one appearance of Geoff Hoon at PMQs before and he got into a mess so I can't see anyone rushing to make him the regular substitute for Tony Blair!
As for who stands in for Blair at PMQs while he's abroad, there's nothing to stop a reversion to the old system, where the Leader of the House stands in.
So John Prescott is not at all "a heartbeat from the premiership". This can only relieve the country!