All of the build details except for the lack of seat stay caps and no gear lever braze ons jibe with the description (posted on Wool Jersey) in a Dec 1977 article on the Iikeston 753 machines. No doubting it's a SB bike, "56" is the frame size and I am not sure what H142 means but it may be a special order. Actual team bikes had a similar code specific to the rider they were made for. SB 1016 would mean she's about a 1976-77 so that fits, too. I suspect she's not 753 but 531 judging from the seat pin size and lack of caps on the seat stays.
You might wish to bring this machine to the attention of the 160-odd Ilkestonisti belonging to my Yahoo Group on Raleigh Team Pros....
Washington DC USA