I am keen to start Home brewing but brewing 23 litres at a time seems a bit excessive, so I'd like to halve the recipe and only use half a can of brew kit. Does anyone know if this works and how long so you store the kit once it's been opened? Thanks
Yes, you can make half a batch.
You can keep liquid malt extract well sealed in the fridge for a few weeks, then it will start degrading. It will loose its freshness, but shouldn't spoil. If you plan using it within 2-3 months, you should be fine.
This Post regarding LME storage, suggest that keeping it frozen is also a good option.
This BYO article mentions that you can keep opened liquid malt extract for a maximum of 3 months:
Liquid malt syrup that has been opened, or is sold in bulk plastic containers, has a much shorter shelf life [than DME]. While buying malt syrup in this form is normally less expensive, it must be used in less than 3 months, or the syrup will degrade or even grow mold. If you have malt syrup in this form, the best way to store it is in the fridge. Pick the smallest possible container to eliminate airspace and avoid oxidation. While the extremely high sugar content inhibits yeast activity and molding, over a three- to four-month time period the syrup will lose its freshness.
P.S.: Making half a batch, is as much work as making a full 23L batch. Knowing that you can store your beer for a few months, 23L isn't that much after all.
What Clinton Green said - making a "half batch" is as much work as making a "full batch". You will also find that splitting a container of liquid is difficult. Cool storage of opened containers shouldn't be an issue for a few months. Aging is reduced in half for every 10°C cooler you can store something.