Complex sentence: Thompson had taken the opportunity to rebuild the first of Gresley's A1 Pacifics, "Great Northern", to the LNER Class A1/1, so defined after Arthur Peppercorn had designed and constructed his own Class A1s in 1947.
Split into simpler sentences: Thompson had taken the opportunity to rebuild the first of Gresley's A1 Pacifics, "Great Northern", to the LNER Class A1/1 specification. The A1/1 was classified in this manner after Arthur Peppercorn had designed and constructed his own Class A1s in 1947.