As has been said, the "granny" charger runs at 10A, which is about 2.4kW at the appox. 235v UK mains voltage. This equates to about 7.5 - 8 miles per hour charging rate (this is what I've measured on our car whilst we've been on holiday, using the granny charger every night). If you use a long extension lead, you'll drop a few volts across the lead, so reducing the charging power a small amount (maybe 5% or so). Therefore the rule of thumb is to use as short an extension as possible, but it's safe to use any mains extension rated for 13A, provided it's fully unwouned (if it's a reel type) and laid in free air. Bear in mind that although 10A may be well under the max. lead rating of 13A, the charger will be drawing this current continuously for several hours, so the extension lead will warm up. Therefore use the best quality extension you can find (with the thickest cable)... some cheap ones are rated at 13A, but use inferior cable and will warm up more, and drop more volts across the cable.