We tried the obvious things with reseating SIM card, factory reset etc, even swapped the SIM for a new one at local Telecom store. No luck.
Oddly it does occasionally show signal in some other areas, but calls tend to fail even then and signal often drops away again soon after.
We took it to Telecom store who have sent it to Telegistics. After examining it Telegistics have claimed "liquid damage" that voids the warranty (with no mention of the actual fault).
The only liquid damage they cite is corrosion on the data port contacts (in an uncovered external port) and on the SIM card contacts.
I've had their technical manager re-evaluate it and he came to the same conclusion. And on a subsequent call they've added that "due to corrosion on the data port we're unable to reflash the system ROM which may help resolve the issue" - which seems absurd given the data port was working fine shortly before the phone went in for service.
I'm currently waiting to hear from the Telecom area manager as I've stated that we consider that a phone that becomes corroded enough in normal usage to void warranty is not fit for purpose.
The only photo available on Telegistics site is hardly damning on the issue of corrosion I don't think.

The staff in the Telecom store have been helpful (and one mentioned he'd seen exactly the same fault previously). Unfortunately my experience with Telegistics hasn't been good - they're terse, a bit argumentative and unwilling to really provide any insight into their evaluation. It really seems like they've opened the phone, immediately looked for anything that could possibly void the warranty and then stopped as soon as they find something.