Complex sentence: It stopped commercial production in 1946, but is now in working order again, having been restored by the Friends of Heckington Windmill, opening in 1986 and is open to visitors in the summer as of 2004.
Split into simpler sentences: It stopped commercial production in 1946, but is now in working order again, having been restored by the Friends of Heckington Windmill, reopening in 1986. The mill is open to visitors in the summer as of 2004.