I would say dont fill it up with hardcore or anything that you dont know from where it is coming. Start some compost heap with shredded leaves, cardboard etc. Also when you will dig up and create borders at other part of garden, use the dug up soil/grass etc to slowly fill it up over time. Also use large pots for annuals/veg growing and you can throw used up compost and end of season plants there.
Posts
As my dear late Mother would have said : " Act in haste, repent at leisure"
There's no rush. I'd advise living with it for a while and having a look at other gardens / books / magazines and see what tickles your fancy.
I would say dont fill it up with hardcore or anything that you dont know from where it is coming. Start some compost heap with shredded leaves, cardboard etc. Also when you will dig up and create borders at other part of garden, use the dug up soil/grass etc to slowly fill it up over time. Also use large pots for annuals/veg growing and you can throw used up compost and end of season plants there.
Does the slope continue down beyond your boundary? If so, you won't be able to'fill it in' without building some kind of retaining wall.
Do the neighbouring gardens have a simlar drop? As Posy says, it may help with drainage issues.
Also, as there appear to be trees/ hedging growing there, you can't build up above the current soil level without jeopardising their survival.