I'm sorry, but there are thousands of cacti - we need to know which one you have to give good advice. If you don't know which one it is, please post a photograph.

If you haven't watered it for this length of time then give it a small drink now. it must be really dry by now. stand the pot in a saucer of water for about an hour. then remove the excess water.

I assume you're talking about the prickly type in which case you should have been watering it freely all summer and be easing off now.

Hywel will know.

If it is starting to flower now I would guess its a Christmas cactus. - does it have a lot of what look like flat leaves growing out of each other? If so and it has begun flowering you should water it now - the flowers will probably be over by Christmas. (Mine are all in bud too) Once the buds have appeared do not turn the plant round or the buds may fall as they try to turn to the light. While its flowering you can water it whenever it gets dry. When its finished give it some liquid feed and continue to water until the end of summer, when you can let it dry for a couple of months. This plant is happy with an occasional soaking and drying out in between rather than lots of small amounts. With experience you can see from the shine on the stems (or not) when it needs a drink.

I would repeat what Steragram has said :)

Some species of Pincushions (Mammillarias) will start to bloom this season, and would need far less water than a Christmas Cactus (Zygocactus). Many species and hybrids of Aloe--often mistaken for cacti--will also be beginning their bloom cycle.

I don't think they bloom now in the UK Tug - I've never seen that but that doesn't prove anything....

I've never seen a desert cactus flower in the autumn here. We have different conditions.
I have an Aloe in flower now though.

Many old gardening books complain of "season reversal" on aloes from South Africa, thinking that it is caused by them being from the southern hemisphere, and unable to adjust their internal clocks. What the writers of those books forgot is that they come from Mediterranean climates, and that many species from north Africa do the same thing, for the same reasons! Where those species come from, the wet season is in the winter, so that is when they grow and bloom. Most of the autumn and winter blooming species of Mammillaria also come from a Mediterranean-like climate in northwestern Mexico and southern California.

People with desert cacti here water them in spring and summer. They are kept dry in the autumn and winter, because it gets too cold.
That's why they don't flower in the autumn.
Watering begins again in the spring, so that's when they flower here.