What i eventually worked out was:

str( (sum(val(substr(i_trn,1,at(":",i_trn)-1)))*3600+sum(val(substr(i_trn,at(":",i_trn)+1)))*60)/3600, 2) + ":"+ right(str(100+(sum(val(substr(i_trn,1,at(":",i_trn)-1)))*3600+sum(val(substr(i_trn,at(":",i_trn)+1)))*60)%60,3),2)

This unwieldy mess works, but i'm going to see what i can do to prevent its use.