First of all Jamie, are they geraniums or are they pelagoniums which are mistakenly called geraniums. If they are the former, they are a hardy perennial and can be left to their own devices in the garden. If they are pelagoniums, they are also a perennial but are not hardy and will be killed by the frosts.
If they are pelagoniums there are two things you can do, either pot the whole plant up and bring it in the house, somewhere warm and light where it will carry on growing over winter although it will tend to grow 'leggy'. Or the method I was told by someone with a lot more knowledge than me and which works for me.
The way I do my pelagoniums Jamie, I cut all tops off to about 3 or 4 inches and trim all the roots so there is only a ball of roots left. I then re pot them in a mixture of compost, sharp sand and a little bone-meal and just keep them damp enough not to be bone dry. The storage is a bit of a problem, they need to be kept cool but frost free so in this weather the greenhouse is a no-no. I keep some in the unheated conservatory and some in my bedroom which is also unheated (purposely for the sake of my seedlings). The plants will put out loads of new shoots and continue to grow throughout the winter. When the new shoots get too big (they tend to be a bit 'leggy' through the lack of light), I trim them back and also remove any dead bits. Come planting out time the plants are well established with loads of shoots on them and if you like, some of the shoots can be taken off and potted on to be grown on as new plants.
As I said, I am no expert, I just know what has worked for me, so if anybody sees anything that is not right in my post please don't hesitate in putting me right.
Robbo.