Hi Robbo,
Only had to do this a handful of times but some of the info might be of use.......
Tools you will need:
set of spanners ( open one end closed the other, ratchet ones are handy but not essential)
set of deep reach sockets with socket wrench and extention bar
selection of cross head and flat screwdrivers largish sizes
stanley knife
set of pliers or grips
clean dust sheets or plastic for laying the sofa down to work on while outside
Tools you should have in case it has wood reinforcement fixings:
claw hammer
chisel
small saw
small pry bar
Firstly underneath the sofa there is usually a hessian type cloth which you can either cut out as it wont be seen when on the floor or, if you are really fussy, carefully remove the staples around 1 long side and 2 short sides to get access to the inside of the sofa.
The sofa picture looks like it can be broken into two halves one being the corner piece the other the arm and two cushions/seats. From underneath remove the bolts that are holding the two main box pieces together so the sofa is now in half.
If you are very lucky once split into two halves they might even go into your house without any more dismantling as they should now be the size of two small sofas. Always worth a try.
If not you can proceed as follows:
If you look underneath the straight half with the arm you should, with a bit of luck, see it is bolted together. If so there will be three or four bolts holding the arm on to the main box/bit you sit on. Undo these and remove the arm from the unit. Next remove the bolts that hold the backrest part of the chair to the bottom (box) part that you sit on. Some of these bolts may be in difficult positions but keep finding them and undoing them untill it all comes apart. NEVER FORCE IT !! If it will not come apart reasonably easily there may be some hidden screw or if you are unlucky a bit of glue holding it.
When you have the back off you will have 3 main parts of that section which should easily fit through your door ( the arm, the back rest and the main seat part )
With the corner unit part I have not had much experience but it will probably be the main seat/box part and the backrest which can be seperated into two pieces. It may be easily done or may be more complicated but you should be able to see what is involved once you have a look underneath.
As with any advice I have given freely, It is followed entirely at your own risk and I am not liable for any resulting damage to property or health.
I hope this helps you in some way.
p.s. what area in london do you live, or do you still live up north ? If you live near me I could maybe come and laugh, sorry I mean give you a hand !!!!
All the best,
Andy.