where are you connecting from, at New Street, or is that where you'll be entering the rail system? because the lines to and from Plymouth and Edinburgh are on the same side of the station, these expresses typically use the higher platform numbers - but not always. New Street typically operates on a lastminute.com planning system so any platform advised now will be out of date tomorrow.
on your mobile, go to
http://ojp.nationalrail.co.uk/service/ldbboard/dep/BHM
and when the 1103 train is shown on that page (typically 30mn before), the platform will show on the LHS.