In reply to TheDrunkenBakers:
I seem to recall the path on the eastern side of the lake is a bit manky for a mile or so due to muddy banks and tree roots, so you might have to walk/scramble for a bit. From memory, heading clockwise it's good from Keswick, past the theatre on the lake, round friars crag, then goes inland a bit. There's then a couple of dodgy bits, then once onto the fields opposite shepherds crag it's a good path across the south end of the lake and over the footbridge, (as mentioned earlier, no need to go to grange). Some nice paths heading north up through the woods on the west shore, although theres a few paths, some more direct than others.
Maybe someone can correct me on this, but to get back to Keswick I think you'll have to head up to Portinscale, across the suspension bridge and then back on a path through the fields parallel to the road to Keswick. I don't think theres a bridge closer to the lake.