Robin,
This is the first time you've mentioned removal of the oil pump by local shop. As you say the bike ran fine for a few hundred miles before their work. I have to agree with Jeff and Alan, something they have done (but I can't say what) is likely to be the root cause of this issue. OldSwartout has suggested some possible causes. Take it back and give them a good talking to !! Maybe, print this topic and show them.
Question - did it previously run fine all the time with the after market fuel tap, not just when it was cold.. Brian.