Best place to purchase the keyboard is from eBay for under £20. The keyboards on this model are stamped into the aluminium with rivets, so you have to pull the keyboard out, then pull out the rivets using a wire cutter. This will leave holes for you to secure the new keyboard with small screws (search "a1369 keyboard screws" on eBay". Before pulling the old keyboard out, plug in and test the new keyboard to make sure it works, and is a keyboard issue, since it could be caused by a damaged keyboard connector on the trackpad.
Just follow the guide on ifixit to remove the logic board. Then you have to remove the speakers by sliding a thin tool (painting knife) under them, do not pull out the speakers. Then remove the black keyboard backlight (depending on model), so you can access the keyboard. Remove the outer screws and then rip the keyboard out.
It is easy to mess up pulling the rivets out of the casing, you have to have a tight grip (not too tight, otherwise you will cut clean through the rivet), then tilt the wire cutter to pull it out. If it slips and damages the rivet, grip it again and rotate the rivet until you see the rivet starting to rotate, then you should be able to tilt and pull it out.
On Youtube, search "a1369 keyboard replacement", will be videos of people doing this.