Platform 1 was in a different location in the old station - roughly where platform 2 is now, but there had not been a platform 7 for many years.
I recall reading that although Thameslink trains will not be calling at London Bridge until May, some trains may be routed that way before then (rather than the slower route via Tulse Hill) for driver training purposes. Likewise the odd Peterborough service may find itself at St Pancras Low level instead of Kings Cross.