Presumably if Lowells have bought the debt off O2, then they will have been handed all your file on this matter, so they will have all the correspondence (or at the very least, a note of the date when reminders were sent, and to which address). If you really want to pursue it further, you need to ask to see that evidence.
All you want to know is that your debt is cleared - if you apply for a copy of your own credit rating reports (OH will have to do his own) then the cleared debt should show up on the report. If it doesn't, you can write to Experian (or whoever provides your report) to get it changed.