The service doesn't always (officially) run to T5, but it uses the opposite platform to the HEX, typically on the right side platform where HEX is on the left. But at weekends and late on then it's barely any slower than HEX and is a sensible option. Otherwise for most of the day you collect it at the T2, T3 HEX station, where it usually uses the same platform as HEX. You can use the Oyster tap in at the T5 Hex station, it can join the dots so long as you also tap at Paddington too on the appropriate platforms. In other words if Paddington have left the gates open you best tap there anyway.