Further to my earlier reply...
I have just called the DVLA
After 12 minutes of pressing options, I was put through to a very rude and unhelpful woman (who wouldnt give me her name for security reasons) who said that if you are not permanently exporting your car to Africa you still have to pay for a UK tax disk. She said failure to do this would result in a fine of up to £1000.
My car was out of the UK for three years. I was fined £35 for not having a tax disk and was told that there was no time limit, after you have paid the fine, for re-taxing the car.
So I paid only £35 for three years of no UK road tax. And had no problems when I came back to the UK. Just went into a post office on day one and bought a new disk.
Just make sure there is someone at home to pay the fine when it arrives.
Or fill out the sorn form online...I dunno