I grew two of these in the same pot after watching Mr. Don, feel quite pleased with myself but can't remember what I am supposed to do when the flowers die off, can anyone tell me ....
Autumn is coming. I don't know where you are or what variety you are growing. Some are much hardier than others, and plants in pots are more vulnerable to frosts than those in the ground. Unless you are somewhere that has reliably mild winters, you should begin thinking of somewhere to put it through the coldest months. I'm in the chilly North and a cold greenhouse (kept just frost free) is enough. A shed or a cool room or porch would do too. They are lovely plants and I am already quite attached to mine!
If you want more plants, don't deadhead but let it go to seed then collect them and plant them up. It takes a while for flowering plants but you get more for free
All bases seem to covered by previous answers.
If you can't get it under cover in winter, if you're in a chilly part of the world, try to get the pot against a warm south facing wall of the house if that's possible. That'll give it a bit of protection.
Do you know which variety it is? It's a beauty.