A reliable backup source for tasks is below. Remember that a sudden new task could be sorted within an hour of first launch so other airspace users could well be en-route when a change happens. Likewise a task can be cancelled at any time, including after the start (if exceptional safety reasons exist). Generally the task area is not changed hugely unless the weather takes a sudden change from predictions.
Soaring Spot :: 19th FAI European Gliding Championships - Lasham 2017
Another good pre-flight tool is
Spot the gliders!, the RAF are a fan of this and I'm told use it before booking out.
Planesandthings