why have my geranium leaves got rusty spots then curled up and died

They have pelargonium rust and are best discarded..make sure you give the pots a good wash/disinfect and start again next season....

they've got rust, I'm afraid, mine have it too currently. You can spray with fungicide, but its too late in the year to do it now. If you're going to keep them (I take it you mean summer bedding geraniums, i.e., pelargoniums, not Cranesbill Geraniums) cut them down and replace the soil in the containers you had them in for next year if you want to grow them there again.
Whoops, posted at the same time...