A1 Steel are in Saville ROAD, Westwood, not far from the A47, on the western side of the city.
B & M Steel, (Barclay and Mattheison) are on the eastern side of the city, not far from Oxney Road.
Take your choice, Both are quite helpful, although I prefer B & M,despite being on the opposite side of town.
Since you seem to be in the Peterborough locality, have you thought of joining the Peterborough Society of Model Engineers? You would get a lot of help and advice if you need it. Look at their website!
Howard