Are there really many gliding sites with 1000 metre runways and 1000 metre cables? and at what point on the winch launch would the cable be released? Surely there must come a point during the launch (45 degrees?) where the cable begins to drag the glider back down towards the ground rather than up in the air, in which case a cable release at say 45 degrees would cut the height risk down to say only 1500 feet rather than 3000 feet? or am I being thick about this?
Yes, a 1000 meter runway and a 1000 meter cable is no exception. In fact, I would say that these numbers are fairly typical. With that combination you should be able to reach 500 meters altitude easily, and probably quite a bit more if you're experienced and there's a good wind.
Here's a typical launch. It's done in three stages.
Once the winch starts pulling you are airborne in about 20 meters. You then pull the nose up to about 45 degrees. This is rather shallow but that's for safety if the cable breaks. The speed builds up to about 140 km/h in 2-3 seconds. Small wonder with a 600 kg MTOW glider and a 300 BHP winch. A Ferrari accelerates slower.
Once you reach 50 meters in altitude you pitch up to about 70-80 degrees. You then play the back pressure and the speed so that you maintain about 100 km/h. Yes, initially that's a 100 km/h almost straight up. The cable at this stage is almost perpendicular to the aircraft.
After reaching 100 meters (in about three-four seconds) you have reached the safety altitude. If the cable breaks you can now fly a normal abbreviated circuit instead of having to land straight ahead. You keep playing the back pressure to maintain a speed of about 100 km/h.
This goes on until you are almost overhead the winch. At this point the winch operator will throttle back, which you feel in the aircraft. You release the back pressure and then release the cable. The cable has got its own parachute to keep tension on the line while the winch operator winches it in.
The total cycle from one launch to the next on a normal day is about a minute and a half. The launch itself is about 20 seconds or so to 500 meters. I haven't timed this though and it might seem longer than it is due to Einsteins Relativity effects and things like that - your first few launches feel like approaching light speed. If you have a good team and you're launching gliders for a competition, the cycle time can be a lot faster.
Depending on the wind, at the site where I flew from even us novices regularly managed to reach 500m in altitude from a 1000m cable. The site is marked as '2300 AGL'.
After the launch it's normally an immediate left or right turn towards the nearest good-looking cloud to see if there's lift there. Beginning glider pilots normally stay rather close to the field, looking for lift, at altitudes up to the cloud base. But always within gliding distance of the field. I think the regulation calls for early students to remain within 5 km of the field. More experienced pilots venture farther away, obviously.
Oh, and glider pilots fly a normal circuit just like powered traffic, although obviously they are descending slightly. They will never, ever, fly over their own launch site, for obvious reasons, unless they are well above the marked altitude.
As others have said, drive up to a gliding field and watch the launching for a while. Have a chat with some of the guys there. You'll find that, due to the nature of gliding operations, there are always plenty of people on the ground helping out and waiting until it's their turn. In fact, unless it's very busy, a trial flight can usually be arranged on the spot and will cost you somewhere in the neighborhood of 30-50 euros.