Mel, your U.K. zone is what, ....your method for keeping over geraniums in temperatures of 5 celcius is fine for storing them...but not what I think Chester has in mind.

If I'm not mistaken Chester, you wish to bring your geraniums in from your garden....pot them up and keep them going as houseplants.

That is entirely possible.
You should dig them up, take them where you can remove the garden soil...no need to wash them....but before re-potting, cut them back by at least half...more if you wish.

Remove any damaged or weak branches and leaves, tease the roots open more and pot them up.
Into a pot with fresh new potting soil (you can mix it with peat moss and coarse sand if you wish to improve drainage)
make sure to keep the soil up away from the drainage holes.
Clay shards work great. Water to the point of drainage in the saucer below. Allow full drainage, then dump the excess in the saucer. Then, no further watering until new growth is evident. Allow the soil to dry down between waterings and always, when you water, do so until drainage is seen. Then dump the excess.

Do not feed until new growth is evident, then do so at 1/2 rate 20/20/20 every 3rd or 4th watering.
If you use liquid fertilizer, water first, then apply fertilizer.

Turn the plant 1/4 turn every day so that all parts of the plant receives equal light.

As the sun diminishes as we get closer to the winter solstace (December 21) you may wish to supplment light by giving it artificial light.
Until mid February when the light gets better the plant will not grow much.

If you have a place in your basement that is kept cooler than the rest of the house...in the range of 40 to 50 F...
(5 to 10 C), you can keep your geraniums cool and dry until mid February when they can be brought back out, cut back, repotted, given much better light and they will re-foliate and eventually re-bloom.

Its your choise....do it now......do it later.

My question has to do with storing geraniums over the winter. Through the largess of the flower gods I may acquire 70 bright bushy scarlet geraniums. My goal was to store them for next spring (and the glorious masses of color thatI could never afford!) I assumed that I would trim all flowers, most of the foliage and clear the roots of dirt. I would them put them in the basement NOT under light in a black plastic bag. I would then remove them from the bag in March (I live in chicago) and allow them some light and maybe spray a weak foliar fish emulson to give them a start while still in the basement and plant them in May. Am I on the right track? What can I do to keep them dry and reduce the chance of mold?

If your garage stays 36 to 38F during the winter I wouldn't put them in the window. I'd put them somewhere where it is dark or even cover them with something so it is dark, and let them go dormant for the winter.

Take off all the flowers first. In the spring as the weather warms and it gets warmer in there, remove the cover. Give them a little water--not too much at first, and then gradually move them closer to the light and gradually give them more water. After a couple of weeks hang in the window until you take them outside.

As the foliage comes, begin to fertilize...1/4 rate to begin. Always, water the plant first, then fertilize.

This all depends on whether the basement is cool enough..dark enough, and DRY. Cool...in the neighborhood of 40 - 50ºF...5 - 10C....dark, so it doesn't initiate growth during the winter, and dry because we do not wish to invite mold or mildew.

The geraniums are put away into such environment either on a shelf or in a paper bag so as to catch the debris that always falls.

If not this way, then into a sunny window goes the plant and keep it going as long as you can....the sun will go down in intensity and the plant will probably stop blooming anyway... blame it on the sun. You can continue to treat it like a houseplant.
At that time, you can treat it like above....
or take cuttings and start many plants from the mother.
Always, take only cuttings from a healthy plant...never from a sick plant.

The storage method can be done annually for as long as the plant stays healthy. Never allow a geranium to be hit by frost before it goes indoors. A frosted plant will probably not survive very long.