Complex sentence: When he found he could not draw the German ships away, Tyrwhitt had returned and engaged the six light cruisers and escorts but broke off the action when seriously outgunned when the battlecruisers returned.
Split into simpler sentences: When he found he could not draw the German ships away, Tyrwhitt had returned to them. Initially, he engaged the six light cruisers and escorts, but broke off the action when seriously outgunned after the battlecruisers returned.