Site name and features
To reach Thorney Island leave the A27 at the A259 (Emsworth) exit and follow the A259 east to a roundabout in the centre of Emsworth. Here go ahead down a slope to water on either side then up an equally short slope to turn right to Thorney at the brow of the hill. Drive south past houses on the left and the marina entrance on the right, then turn left into Thornham Lane (just as the open marshes come into sight). Park at this junction (MARKED "1" ON THE MAP). If you continue down the main road you will come to an Army guard room and will not be allowed further unless you have a permit.
From the Thornham Lane junction walk west back across the road and climb the stile by wooden gates on the north side of the hedge ahead (climbing the gate south of it and using the clinker road is tresspassing). Continue along the north side of the hedge through the farm yard (beware deep manure) to the west seawall (there is a longer alternative route by going back up the road and into the marina, forking left as you enter it, to reach the seawall further up). You can walk south freely along the seawall past the Little Deeps to the Great Deeps, but beyond them is a military fence - to go through follow the instructions below.
At the fence (MARKED "2" ON THE MAP) press the intercom button and wait for a reply - you may have to wait a few minutes if the guards are busy with a vehicle on the road. If you get no reply press again after a reasonable interval. The guard will eventually reply and want your name and address and will then operate the switch to undo the lock (be ready to push the gate when the lock clicks) after warning you to keep to the coatal path all around the military area. The military reserve the right to refuse access but I have never heard of it happening (though the intercom does not always work!)
The coastal path takes you all round the island. At the south west point is Pilsey Island which is also off limits but at winter high tides there is usually a substantial roost of mixed waders on the sands west of the island and/or on the airfield. As you return north up the east side of the island you may find the shore route blocked by the high tide in which case there is a marked footpath leading in round the buidings to the road which brings you back to the shore (if the tide is very high you may have to go through the churchyard and climb its wall back to the shore)
Back at the east end of the Great Deeps (MARKED "3" ON THE MAP) you use the same procedure for getting back through the military fence, then continue up the seawall and turn left along the first track which soon becomes Thornham Lane and takes you back to the start. (You can of course go round the island the opposite way to that described here.)
All land south of Thornham Lane is private - the fields between it and the Great Deeps are not open to the public but the field north of the lane at its east end is open but is reverting to saltmarsh. Several public paths (marked in yellow on the map) are open to the public.
There are two carparking places to access this area - marked "4" and "5" on the map. We will start with the one marked "4" in Farm Lane, Nutbourne. To find this follow the directions for Thorney Island but continue along the A259 past the Thorney turning and on through Southbourne. Beyond that village there is a part straight stretch of road with the Weston Apple Farm orchards on your right. After this ignore a tiny road (School Lane) on your right but shortly after it turn right into Farm Lane (also a very minor road) and park outside the farm as soon as the road widens (before coming to a meadow on the left). This is not an official carpark and space is limited - do not block the road (and avoid winter Sunday mornings when a regular 'bird watching for beginners' group meets here)
From Farm Lane you have two main options - one is to make a short (but worth-while) circuit along the shore to Prinsted and back through the orchards. the other is to make the much longer circuit of the Cobnor penninsula. In both cases you can start by looking for a Little Owl that has in past years inhabited the farm buildings (but may have been driven away by the contruction of new stables and much more human activity in the buildings) and by viewing the marshy meadow on your right as you walk the field path to the shore. The stream in the meadow is the Ham Brook and meadow itself has I think been acquired by the Chichester Harbour conservancy and its 'marshiness' much enhanced to attract wildfowl.
On reaching the seawall the short route turns right and follows the route shown on the map, turning right at the first and all subsequent path junctions to return through the orchards (often full of Fieldfare and Redwing in the winter) to Farm Lane.
To go round Cobnor you turn left at the seawall and follow the shore down the west side of the penninsula, then follow the marked path through the Activities Centre back to the east shore (the road north from the Centre is not a public path). You have a number of options for your return route with not much to choose between them - make up your own mind!