Thank you for your question. I understand your concern. This should absolutely be evaluated and worked up. I suggest you start with your internist or GYN. Best of luck.
I would see your GP and your PS. You may well need some imaging which would be best through the NHS route rather than privately via your PS.
I have now found quite a large hard lump behind my nipple. Is it a worry?
Yes, I would say that any newly found breast lesion or "lump" warrants evaluation ASAP. I would suggest that you start with your primary care doctor, Gyn, or internist. Best wishes.