It would depend on many factors, Kate: size of plot, number of plants, how windy/shady your plot is, annual rainfall etc, but in my opinion a 1000 litre tank (one cubic metre) is way too small. You would need to give 20 litres per plant (deep watering) every 2-3 weeks over the summer months unless there is rain. Vegetables are thirsty and would require frequent, possibly daily watering. You could wean trees/shrubs off water after a while, depending on your conditions, but even some of these will require the occasional soak. We have a 30 cubic cistern and it is certainly not enough (however, we do have 600+ plants). With our annual precipitation of 350mm, it gets filled from roof run-off during the winter months, so a 1-cubic tank will fill up very quickly and most of the rest of the water will be wasted. There is no comparison with London conditions. We rarely use all the water from our water butt in London.
I would suggest you have the biggest cistern you can afford. You will not regret it.