Nobody here can advise you whether it is safe to take a complementary medicine such as CBD alongside cancer drugs such as Letrazole as it may have contraindications for you which we wouldn't be aware of. Your Oncologist will be able to tell you whether it is Ok to use so you really need to speak to someone who is involved with your treatment, your Macmillan nurse should be able to direct you you to right person even though they aren't a doctor and can't give you that advise personally.  Currently the NHS don't recommend it for pain relief so you may be better to seek further help to manage your pain other than through supplements. If you can't speak to someone involved in your treatment maybe you could try talking to your GP. 

I'm sorry we can't offer any further advice, as you can see from the NHS guidance, there really isn't any support for using CBD oil in this way and so it isn't something we have any further information on www.nhs.uk/.../

Good luck and best wishes