3kw cooker - 20amp fuse and 4mm cable?

 I have a 3kw dual fuel cooker installation in a couple of weeks and
have discovered that I do not have a dedicated circuit in place for
this (the existing small oven is plugged in to a standard socket). I
realise I need to run a cable from my fuse box, and hence have carried
out some research... I came up with the following:

3kw appliance = 13amps, so if I use a 20 amp fuse at the consumer
unit, I can then use 4mm cable which seems to take btwn 25 and 30
amps....

I thought I had this all sussed, until I spoke to someone who
said.....

3kw appliance needs 30amp fuse and 6mm cable. Is this correct or just
erring on the side of caution??

The run is about 19 metres, and will be under floorboards, except for
a couple of metres of trunking down the wall.

Any advice greatly appreciated!

  Mark Heathcote wrote:

<SNIP>

NOTE to US readers: This is a UK-specific problem and the OP
has posted to the UK group.