The first thing to bear in mind in an EFATO is survivability, not saving the aircraft. That has to be sacrificed if necessary. So leading on from that, most airfields will have somewhere ahead to achieve the survivable crash!!! If there really isn't anywhere ahead, as seems to be the case at Sywell, then perhaps a special procedure should be devised.
On the safety issue though, the turn back should never be considered below 500ft.
But why!! I here some cry!
An experience I had will illustrate why. I have a few hundred hours gliding experience from the distant past, and one incident in particular I remember well. I had a winch launch failure at 400ft, but as the launch speed was slow all the way up, there was insufficient airfield ahead to land in. I had foolishly hung on to the cable hoping that the damned winch driver would open the throttle more. The though that the winch may not have been developing enough power hadn't crossed my mind. It was a thermic day and I wanted UP!!!
I elected to turn and land back on the airfield downwind rather than ahead into a field.....Pride was at stake. So, decision made I made the turn, and what followed frightened me:
At low altitude you will get a distinct impression of speed from seeing the ground so close. With the initial headwind it all seemed so easy, but then came the fear! As you turn through 90 degrees and across the wind the ground seems to be moving very fast, and there is a tendency to pitch the nose up thinking the speed is too high......Fatal. I knew I was slow in the turn but fortunately I had enough experience to resist the visual cues and fly by feel, and I pitched the nose down again.
So, I accept I was foolish to turn back (even in a glider), but I managed to land back and by using the available width of the grass airfield minimised the effect of the tailwind by landing more across the wind. BUT, I was pushing it wasn't I? I estimate that as I turned through the crosswind I was very close to the stalling speed, and we all know what happens then!!
I believe now that apart from the height loss in a turn, one of the main dangers is the visual effects close to the ground and the consequent urge to pitch up because of the impression that the speed is too high.
I really learned from that experience, and I'm definitely in the EFATO land ahead camp, unless there is really no alternative (Sywell perhaps) or there is plenty of height to play with.