This is a very common fault on your appliance.
The defrost cicuit isn't working properly, this is the reason for the icing up.
Normally, a new thermal cut fuse, defrost sensor and evaporator cover (with fan) sorts it out.
You may also find the defrost heater or gutter heater has failed.
You would need to test the heaters to see if they've failed.
If they have, they'll need replacing.
There is a full kit, including heaters, available on Ebay for around £88
The complete kit, fitted should sort out any problem you have on the defrost circuit.
Alternatively, if you don't want to try the diy route, British Gas have an offer on at the moment.
£125 inclusive of parts and labour.
If they can't fix it, there's no charge.
I believe they reduce the price if you also put your appliances on contract.
Click here for the British Gas offer
Check out the heaters on your machine first if you go the diy route.
You'll need to do a full defrost of the machine first so you can get the panels off to check the components.
Hope this helps
Get back to me if you need more help or information on this fault.