Firstly, don't panic. I would question why there was a fence put in a couple of metres from your back boundary line, unless the previous owners also didn't know what to do with the slope and decided to conceal it. Unless you are looking at digging large holes in your garden and using the spoils of soil to fill in the slope, it will be costly to buy soil to fill the void and bring it up to the same height as your lawn. I would agree with the others and plant a hedge along the back. If you plant a mixed native hedge and kept on top of the pruning you could have colour from leaves and flowers for 9 months of the year, ie Hawthorn, Blackthorn, Crab Apple, Dog Rose, Dogwood, Guelder Rose, Hazel and Spindle. I would plant it close to the back boundary, i.e. 90cm-1m away. Such a hedge is excellent for wildlife. Then, in front to the left I would plant Prunus subhirtella Autumnalis. This is an ornamental cherry tree and will flower for the remaining 3 months of the year. You can then create a 'woodland' hedgerow garden towards the end. This doesn't mean leaving it and letting in brambles, thistles and stinging nettles, but planting primulas, polyanthus, forget-me-nots, fritillaries, ammi majus, ammi visnaga, lamiums, dicentras, ferns and Solomon's seal. These would fill the space, create a great wildlife garden and produce cut flowers for the house. Then focus on the lawn. If the lawn is really bad then lift and either lay turf or for a cheaper option seed it. But first, if there is grass and it covers most of the lawn, feed it, kill the weeds (or All-in-One feed and weed) and aerate it and see what happens - it may just need love. So many people pull up lawns when most lawns will bounce back if cared for. I hope this helps. Enjoy your garden. Mark