You'll have a hard time finding ready made pendants in that length. You don't want to replace the whole assembly anyway, besides which changing the ceiling portion is likely to involve reconnecting quite a number of wires inside (unless you happen to have the type where the pendant simply unplugs, but they're not all that common so I'll assume you have the regular type).
Changing just the cord itself is not at all difficult and just involves two wires at each end. All you need to buy from B&Q or some other suitable store is whatever length you require of PVC-insulated, 2-core 0.75mm flex. They should know what that means.
Step-by-step replacement:
1.
Safety first: Turn off the juice to the house at the main switch. Turning off just the light itself at the wall can still leave live wires inside the ceiling box, so although I'll change cords like that myself I'm not going to recommend that you do so. Shut off everything at the main to be sure.
2. Unscrew the cover from the ceiling box. This is often the hardest part if it's been painted over a dozen times since it was installed. You may have to run a Stanley knife around the upper edge to clear the paint.
3. With the box opened, you'll see something like this:
Yours might be arranged slightly differently inside, but most modern fittings are similar. You may have multiple cables coming in from above the ceiling as well, but that doesn't affect what you're doing.
4. Unhook the wires from the pendant cord and disconnect them from the terminals. Make a note of which wire went where. Now you can come down to ground level to make things easier.
5. Unscrew the back cover of the actual lampholder. You'll see two little terminals inside where the cord connects, like this:
6. Disconnect the cord from the holder, then strip the end of your new cord to the right length to fit.
7. Connect the new cord to the holder, making sure that the copper is held snugly by the terminal screws (but not
too tight!), and hooking the wires back round the little tabs as support, as in my picture above. Some newer lampholders don't have these lugs but instead use a little plastic disk which goes on before the cover. On a regular bayonet lampholder it doesn't matter which way round the two wires go. Replace the rear cover when done.
8. Cut the cord to length, then strip the other end to fit the ceiling box. Reconnect the two wires matching colors with those you removed earlier, make sure that you hook the wires around the little lugs for support, then screw the cover back on. (You did thread it over the cord before you connected it, didn't you?
).
9. Replace the bulb, turn the power back on at the main switch. You're done!
