LHR Transit Times
 

Hi I arrive into T3 and out of T4. 1 Ticket so baggage will go through. It’s a Sunday and Elizabeth line for train transfer only runs every hour! Is 2 and a half hours sufficient time ?

That should be enough. There's a bus in the transit zone to change terminals. You don't have to clear immigration and customs, take a train to change terminals, then go back through security.

When you get off the plane, follow the signs to Flight Connections.