Hi Jenny, I think the injection is the HCG trigger that will stimulate your follicles to rupture and release the eggs. They will only give you that shot if your CD12 scan shows mature follicles (usually 18mm or more). Ovulation should occur within 24-36 hours, so you'll need to start BD'ing away.
Don't have much advice on BD for poor swimmers though. My DH's morphology is not ideal, and my cervix is curved. I also suspect that I have hostile CM though I've never had that tested. That's why I've always paired my Clomid cycles with IUI to have all bases covered.