There's a great book "Great Answers to Tough Interview Questions".
Pick some out at random, read the suggestions and spend time formulating a response that's "you". The idea is that you will not have a script, but that you will have tuned in to interview-style questionning and answer formulation. Always know your strengths and weaknesses and have examples ready but definitely make them palatable.
Ask your DP to role play him being the interviewer and ask the questions - I almost guarantee you will stumble/giggle at the role but at the real interview you will not because you have the stumble/giggle out of the way.
Read up as much as you can about the place of work beforehand- online, or you can ring and ask if they have any literature they could send you.
.....always take a smart A4 leather/pleather folder with paper in or a black smart looking A4 notebook. A4 generally looks more professional and "important" than an A5 (which looks a bit more diary-ish). Take two or three extra copies of your CV (which should be no more than 2 pages maximum, unless it is common in social work to have long CV's. Get your folder out when they get their papers out. It automatically "matches" your props with theirs and makes it seem more like a mutual meeting instead of an interview. It also gives you a focus so you don't feel like rabbit in headlights.
Have two or three pertinent but "safe" questions ready to ask at the end. It is usually a very positive way to end an interview if you can ask them a question because it redresses the balance of them doing all the asking and you doing all the answering - it makes you seem more like "one of them" and also shows you're not afraid to ask questions, also, that you are not desperate to hot-foot it out of there (even if you are).
Make sure your dress sense is appropriate, if in doubt, go conservative . You don't have to be super trendy but every detail must be as perfect as you can make it - nails filed, shoes polished, no runs in tights, no missing buttons, no dust marks on bottom of handbag, subtle jewellery (best to stick to something very simple, no jangly stuff or statement pieces). Hair must be appropriately groomed, makeup subtle (unless you are a make-up artist!) Clean your teeth thoroughly and have some fresh mints just before you go in. I wouldn't recommend accepting hot drinks unless you are very steady handed, but a glass of water is good to have on the table, all the talking and nerves can really make your mouth dry out.
Finally you must have a strong, positive handshake and a genuine "Thank you for your time". I can't say how important it is that you say this clearly, smiling and looking them in the eye too. Make sure you have started the interview with smiles and handshakes, it's as well to have a neutral pleasant comment to make as an ice-breaker (if the interviewer doesn't) but you must keep it short, not a story. Ie they say "How was your journey" does not require the real story of traffic jams and roadworks, it is "Great thank you (always start positive), the odd bit of traffic but that's to be expected!" You would not believe the ramblings some nervous people come out with at the simplest ice-breaker question.
HTH! good luck!