Yes it is 13x250=3250watt!
thanks mate for replying quickly. I went to a shop as there are many extension cords out there. Most of them have 13A and 250V but prices vary. Such as: 1 with 4 sockets costs £10 and another with 4 sockets cost £35. But both have 13A and 250V. Do you think £35 ones better?