After leaving the hermitage of the Queen of the Waste Lands, Lancelot rode at random (Malory does not specify for how long) into a wild forest, where
"at the last he came to a stony cross which departed two ways in waste land; and by the cross was a stone that was of marble, but it was so dark that Sir Launcelot might not wit what it was."
Looking around, he saw an old chapel. The door was “waste and broken,” but within the chapel was a fair altar, richly arrayed with cloth of silk and a silver candlestick holding six candles. Lancelot found no way, however, to get inside, so he returned and slept by the cross.
In the night, when he was between sleep and waking, he saw a knight borne to the chapel on a litter. The Grail arrived on a silver table and healed the sick knight. Afterward, including that Lancelot must be in some deadly sin because he had slept through the miracle, the newly healed knight took Lancelot’s helm, sword, and horse. Lancelot then heard a voice speaking to him in symbols. Waking, he found a hermitage on a high hill, where the hermit (Nascien?) expounded the vision.
The chapel might have been either in or near Listeneise (Listenois). Cross Fell, near the joining of Durham, Cumberland, and Northumberland counties, might be a good place to put it.