Have you ever had a split condom?
Are you aware of PEP? PEP stands for Post Exposure Prophylaxis, it is a month long course of drugs that can prevent the HIV virus forming. However, it is only effective is this treatment is commenced within 4-72 hours of sexual activity. If you have an accident, and most particularly an accident after receptive anal, then you may want to go and consider asking your local A&E, also I think the private clinics may be able to supply it. The side effects are apparently unpleasant, but the treatment is proving effective.
Hope that is useful, i will disappear now!