theres no plan to resignal the Arun Valley in the near future, so I think Pulborough at lease is safe (along with Arundel and Billingshurst) I imagine if nay box were to close then the part time box at Amberley would go
I wouldn't bank on it, according to the Sussex RUS (2009):
"On the Arun Valley the base renewal scheme
for CP4 involves the centralised control of all
signals in the Billingshurst to Amberley/Arundel
signalling areas and resulting integration with
the existing Horsham control panel within
Three Bridges Area Signalling Centre (ASC)."
So that suggests about three years max, ie 2014.