HJP, you will need another pnr. Try logging in at BA MMB and see if you can find it there. If not, call BA. The pnr should be different from the one that you have used previously.

Might be that it won't work in your case, though, since both your flights are BA-coded. In my case the first flight had AY code.

After all the hassle, I'll avoid BA as well unless it's BA all the way and I can check in with them the way it's meant to be.