Complex sentence: With the advent of TOPS, Class 34 had been reserved for these modified locomotives but it was not used and they were grouped, instead, into class 33/1 being numbered in the range 33101--33119.
Split into simpler sentences: With the advent of TOPS, Class 34 had been reserved for these modified locomotives but it was not used. The modified locomotives were grouped into class 33/1 and numbered in the range 33101--33119.