Thanks Rich,
I finally got a response from the support centre, and arranged to have the unit sent for repair. I just want to relate what happened next:
The laptop was sent away, which was a reasonably painless experience. However, when the repair centre got in touch it was only to tell me that a) rather than just replacing the battery, the entire motherboard would need to be replaced and b) this would not be carried out under warranty as there was evidence of impact damage to the unit - the repairs would cost the princely sum of £419.73 (i.e the price of a brand-new laptop). The news of the damage was a shock (clearly I never would have sent it off had I known there was any damage on it), and once I received the laptop back (unrepaired, obviously) I could see that part of the extremely flimsy plastic casing around the hinge area had indeed snapped off.
Now, I know for a fact that the unit had been malfunctioning from the moment I first took it out of the box (without dropping it on the floor!) and switched it on. I had assumed (wrongly as it turned out) that the date/time issue was due solely to the internal battery needing replacement. Any damage to the laptop had been sustained inside the laptop bag (where I found the missing piece of hinge casing), which logically must have happened at a later date than when I discovered the fault (since I had not placed the laptop in the bag before turning it on and discovering the issue). Therefore the problem with the date/time could not have come about as a result of any impact on the laptop.
I can understand that from Sony's point of view the casing damage invalidated the warranty, as obviously I can provide no evidence to suggest that the unit was malfunctioning prior to sustaining a knock (although I suspect any engineer worth their salt would have been able to determine that the damage was localised to the casing only and had not been transmitted to the sensitive electronics - clearly it is not in Sony's interest to carry out such selfless investigations), and once they start accepting such damages under warranty then the floodgates open, that's fair enough.
However, it doesn't change the bald facts of the situation from my perspective:
I have been sold a faulty Sony laptop. Due to an unfortunate series of events (i.e. the casing getting damaged prior to my having had it picked up for repair) I now have to foot the bill for the repair myself, and the repair turns out not to be a simple battery-change (I have checked this myself now by a quick replacement, since the warranty is already invalidated) but something more serious on the motherboard, which apparently would need to be replaced, at a cost that would make buying a new (non-Sony) laptop a far more sensible solution.
On the basis of this experience, and the failure of the support team to respond promptly as described in my initial post, I think it is fair to say I will never buy any Sony equipment again, and would recommend others to avoid them also.
