Posted: 06/08/2016 at 11:52
In theory,if it's had the same treatment as the others, and they're fine, it should also be fine Gemma.
Unfortunately, these things can happen with gardening!
Is the pot raised off the deck? If it's not, it could simply be waterlogged, which Hebes hate. Other reasons could be wind damage, but that looks unlikely as the whole plant is brown and you'd have damage on the other one. Have you got a visiting tom cat? Animal urine can cause browning and lead to the plant dying. My last thought is vine weevil, although Hebes aren't normally susceptible. The grubs do the damage by eating through the roots. If the plant pulls out the pot easily and the roots are a bit non existent, that might be the reason. If so, you might see some of the grubs in there too.
Hope that's a few ideas to think about