Have a look at what the mental health act is. Im not familiar with how it is written in the UK but i know that in Australia, it is written that you are only to hold someone against their will when there is no way to guarantee their safety. In these instances there is a timeframe as to how long they can do that against someone's will (before needed it to go to a tribunal to get he case assessed and a longer involuntary treatment order issues).
As far as i am aware, in most instances at that initial time when someone has attempted and may be taken into hospital (or voice the desire/need to commit suicide) that person can be held if it seems reasonable that they may carry that out for a maximum of 12 hours (without a psychiatrist being involved). If after 12 hours they are assessed to still be a danger then a psychiatrist reviews the person and can extend it to a total of 36 hours in order to complete a full assessment. Be aware that this is the Australian version, so you need to see what the UK is all about.
During that period, the person will be assessed and it will be determined if they need involuntary treatment. This is often not the case as most people will volunatrily agree to reamin for treatment purposes. The only times when someone is kept against their will past this initial assessment period is when they are clearly a danger to themselves, to others or there is no way that they can be reasoned with/understand that they need treatment. This is when it will progress to a tribunal level and that person will be recommended to be held for treatment. Often times, these periods arent too long (1 week or so) as once medications start to work and ther person is feeling better they may be able to go home if they agree for follow-up and to continue taking the medications. It's rare now that someone is kept for weeks to months at a time against their will for attempting to commit suicide (unless they are very unstable and have a lot of other mental health issues). The only times I've seen people kept longer than a week are people who are psychotic or suffer from schizophrenia and have stopped taking there medications. Occassionally there are people with bipolar disorder that may have a slightly longer stay, but usually this is not the case.
I read above that someone suggested you be careful about being AB at home. If your partner/boyfriend/big/caretaker/etc is put under the mental health act it only applies to that person and not others around them. (i.e. dont expect the police to randomly knock on your door because they saw you in a diaper). They dont do survelience at the persons's house or anything like that. The point of detaining someone is to ensure they remain in a safe environment, which means they stay at the hospital.
Lastly, siysiy, try to be supportive. I know you might want to regress due to being stressed from all of this happening. But making someone feel bad or calling him names or a "quitter". Mental health issues aren't something you can just turn on and off, trust me as i know and have dealt with depression for the last 4 years on my own without support from family (i didnt tell them because i was ashamed to). You need to be supportive to help him get better. Creating guilt is not a good thing to do (and from what i have seen, people who feel guilt are more likely to have another episode of severe depression/suicidal ideation and are more likely to be successful as they dont want the guilt feeling to happen again. Guilt is a very powerful motivator and not many people know that it is very much a silent killer (as everyone focuses on the sadness of depression and not the embarrassment and guilt that happens after).
I know you said that "paddy" was a person of interest with the police. This has no weighing in wheather they will keep him for mental health assessment or not. The mental health workers/psychiartists are only interested in the here and now. Wheather he has been involved in the police or not is not something they are interested in. They may discuss that when he gets better but the first priority is to make someone safe and start the healing. Everything else can come later.