I wanted to post so others who may have a similar (male factor) problem and could benefit from our experience.
After few years ttc it turned out that DH has a very low sperm count (~3m/ml), what would be classed as severe oligozoospermia / severe sub-fertility. We were advised by our GP and specialist (reputable) infertility clinic that nothing could be done. It is really unexplained, we should take vitamin supplements, and save money for IVF.
I found on the internet varicocele could cause infertility and - luckily for us - visited a consultant privately who said he thought there may be some varicocele but not much. The consultant said research on varicocele embolisation is inconclusive and most studies say how embolisation is not effective. I went to my GP who was not convinced DH had varicocele at all and quoted research that was against treating varicocele. We managed to persuade them to refer DH to urologist who was uncertain about the whole thing and the benefits of treatment but, given the number of years ttc we had, was sympathetic and referred DH to a radiologist for embolisation procedure.
The procedure was quick and it turned out there was definitely varicocele to fix. After few months DH did semen analysis tests to confirm if anything improved and it turned out that the concentration improved ten fold! It was now at ~30m/ml, and the motility improved slightly too (motility was relatively Ok previously).
So, if you are having male factor problems, varicocele embolisation may be a solution, you have to ask and push for it is not on the standard NHS "menu"!
Best wishes and good luck to eveyrone on the forum