Geetee1972 is spot on – I would set £150 aside from the 1k for a decent helmet, gloves, shorts, a hydration pack, a few tools and if you are commuting some bright lights.
£850 will get you a nice hardtail. Go ride a few, there is a massive selction at that price point; orange, GT, giant, kona, felt, charge, trek, spesh, cube etc etc