Hi Minty.  Welcome to the forum and sorry to hear that you have BC.  I did a google search on drug interactions and actually found a postee on here who was asking the same question back in 2013.  The nurses said that there had been no research into this at that time and that the GP should be able to answer this query.  Obviously you have already gone down that route and he is unable to give you the information.  You may try a chemist too, who may have more information.

If there has been no investigations into the interaction of these drugs, personally I would steer clear of a herbal supplement which could potentially alter the effectiveness of your Tamoxifen.  Your health is really important  and you need to be certain that the Tamoxifen is working 100% to clear your disease.  It would be much more sensible to start a healthy eating plan and have lots of exercise - both of which would lower the risk of the cancer returning.  All the best to you x