We have often camped in October with no problem. This year we camped at New Year and in early March too, which was pretty cold.
As everyone here says, wrapping up is the key. Full thermal underwear is essential, particularly at night, and warm socks. A good sleeping bag, or even better a duvet, and plenty of blankets and a well insulated mattress and you should be fine.
We usually take a butane gas camping heater as well just to warm up the tent while getting ready for bed. If you have EHU, of course, you can have an electric fan heater which is probably even better.
The evenings will be dark a lot earlier of course so if you can go to a site which allows campfires that will probably be more fun and a good way of staying warm all evening.
The most important thing though I think is a warm woolly hat, as you lose most of your heat through your head. I often wear mine in bed and find this is the best way of staying warm all night if not the most stylish.
I did read an article in a Camping magazine about really cold weather camping where this bloke wore a face mask - the sort you use when you are doing DIY to avoid dust - and he said this stopped his face from getting cold where it poked out of his sleeping bag and let him breathe too (always useful). Possibly a bit extreme in October though...