To start off take everything out of your oven including the shelves. Grab a washing up bowl and fill it with hot water and washing up liquid. Take an old long armed washing up brush, dip it into the water and give the interior and door on the oven a good firm scrub. Do the same to the shelves and set them to one side.
Then take a handful of bicarbonate of soda and pat in on (thickly) to the surface, continue until the entire inside of the oven is covered before moving onto the door and shelves and leave overnight. If you prefer you can make a paste of bicarbonate of soda and water in a bowl (so it resembles the thickness of an icing sugar and water mixture a-top a fairy cake) and brush it on with an old pastry brush. Wipe away with a cloth and hot water. If any of the mixture is difficult to budge, use the long armed washing up brush to wipe it away in circular motions.
We prefer not to deep clean our oven because frankly it is a hassle, so we try to wipe down the shelves in the sink after every use. If you clean them when they are still slightly warm you get off most of the dirt, grease and food stains straight away. We tend to have a roast dinner over the weekend and it is at that point that I tend to give the inside of the oven a good wipe down to keep on top of any spills. Who wants to spend ages tackling the oven because the landlord is coming over, when you could have your feet up watching Downton Abbey?