So far so good!
Thank you for all the replies!
I think I am going to have to acquire me an AC amp meter and take some actual measurements.
I must say that this was an area which I never thought about too much - for example, I vaguely thought LCD screens took a lot less than a CRT screen, but they take more than I thought. Likewise, even a modest laser printer takes about 500 - 600 watts - and that's a lot more than an ink jet printer at about 40 watts.
I have also discovered that there is more than one type of 12 volt lead acid battery - car batteries are designed to dispense very high currents on a low duty cycle: for a power backup application one needs so-called "deep discharge" batteries which are optimised to provide a lower current (tens of amps rather than hundreds of amps) over a sustained period.
My original thinking was to simply buy a generator but decided against this because if they're not used all the time they can be extremely difficult to start, particularly the cheap ones where you mix two-stroke oil in with the petrol - over a period of time the petrol evaporates leaving the carburettor full of two-stroke oil. Also, some of them have ridiculously low running time between services - one I looked at needed servicing every 24 running hours! Last but not least, they generate a lot of carbon monoxide - not nice close to a house!
So far I have acquired two 102 ampere hour deep discharge batteries. One of these will power my main desktop system (and the house's internet gateway) by means of a combined 1000 watt inverter (2700 watt surge rating!) with a built in three stage 10 amp battery charger (which tapers the charge rate so the battery doesn't get cooked). I just hope that I don't get anywhere near 1000 watts because that will mean some significant drainage on the battery!
The other battery will power an 800 watt inverter with a separate 10 amp battery charger for Mrs Feline's Apple Mac.
I also have a 150 watt inverter for lap top with 3G card and ink jet printer. With a bit of forethought I can quickly transfer work in progress to a memory stick and continue working (and communicating) on the laptop.
I also have about half a dozen fluorescent lamps with rechargeable batteries, so will be able to see what I'm doing after dark ...
At some stage I will likely also invest in a solar panel to recharge the batteries (we have plenty of sunlight!) - so I think I've more or less taken care of work continuity (important because I work from home - and no work = no income!)
Cooking (sort of) taken care of with bottled gas ...
It also occurred to me that a real neat solution would be to have a couple of rollers coupled to a generator: if you're at home then it's likely that your car will also be at home. Drive car onto rollers and put into gear and one should be able to produce a good few kilowatts!