Yes, you just go to T5B on the train as soon as you're through security.
Go up to the lounge and enjoy.
When it's time to go, go down the escalator to the lifts that are straight ahead, level -4 for the walkway, then walk back to A and at the end it's the lift back into the A terminal. It's very easy.
I always head back to T5A as I'm always flying to Dublin and it's always at an A gate, so I leave early enough then sit in the South lounge for a bit before departure.