Excuse the crude 'text' diagram', but it's much quicker than me mucking about with Paint or Pinta.
20A Fuse ========== Orig Power Skt -- xxA Fuse ------------- 2nd Power Skt
|___Original Factory Wiring______| |____Your Extension for 2nd Skt_______|
Your extension cable must be rated to allow for the maximum current that you will be drawing plus a small safety margin (i.e. about 5A). However, since the upstream (i.e. the factory) cable) is rated at around 25A there's not much point in using cable larger than that in your extension. Ideally try to keep the section of cable between the original socket and the xxA fuse as short as reasonably possible; this minimises the length of unprotected cable.
Your device (say a portable kettle) being plugged into the 2nd socket draws 20A at full load.
The cable must be able to withstand at least 20A continuously so it doesn't overheat and thus catch fire.
Add in a safety margin (e.g. 5A) to ensure the cable is not running at its maximum continuous rated value, therefore a good choice in this case would be some 25A cable.
The xxA fuse (in the extension) must be be greater than the maximum rated value of the cable (25A) otherwise a short would overload the cable before the fuse had chance to 'blow'.
I wouldn't use mains cable as :-
The flexible cable (as used from plug to appliance) is typically rated for continuous use at 5A, 10A or 13A which may well be less the intended load.
The solid core grey sheathed cable (as used from distribution (a.k.a. 'fuse') box to light,socket,cooker) is not suitable for use where the cable is subject to vibration as it likely to fracture internally due to work hardening of the single core during movement.
Maplin don't do any 25A cable, but they do a small range of suitable automotive 2 core cables, 6A, 10A, 15A, 20A.
As for the 3-way adapter :-
The Bay of E add says in point 14d that its a total 60W (i.e. 5A) load which means they should be using 7A or 10A cable (maybe 10A) cable, but for £2 I'd guess the manufacturer has probably penny pinched and used 5A cable.
If you're only running small items, i.e. Sat Nav, Phone Charger, Ipod then a 5A fuse would be safer than using a 10A fuse; just make sure the total current draw of the plugged in itmes does't exceed 5A or 60W (depending on how the items quote their load values).
The plug on the end of adaptor would ideally have fuse built into the tip, but if the picture is accurate it doesn't look like it.
If you need to convert Watts to Amps (only for DC ratings, AC ratings are different) then use this :-
Watts = Volts * Amps
Amps = Watts / Volts
When adding the extension I wouldn't use 'Scotchlock' type connectors as in higher power usage the 'Scotchlock' need to match the diameter of the cable cores properly; they're only a friction fit over a very small contact area (just think how slim the blade is).
A good thing to keep in mind is that a fuse's purpose is to sacrifice itself in order to protect the cable (from overload damage) and thus by inference the safety of your car.