1) Most dental schools require a bare minimum of 2 weeks shadowing at a general dental practice (restorative dentistry). The more, the better. Just a heads up, work experience at any other sort of facility, e.g Orthodontics, doesnt count towards the 2 weeks but looks good on the application.
2) Finding work experience for dentistry is very hard and competitive. Try to do it as soon as possible. in my opinion, its essencial you complete the bulk of your general dental practice work experience before making your UCAS application, so you can write about your experiences and what you have learnt. September is definitely pushing it very late.
3) Like I said above, bare minimum 2 weeks at a GDP. any thing else, e.g carehomes look fantastic so try to get as much as possible. Get involved in your community.
4) Dentistry is one of the most competitive courses period (definitely up there with medicine). Not to put you off, but this past year, applying to dentistry has been one of the most stressful things. You have to score quite high on the UKCAT test to be in for a good chance. However, despite what people say, its not impossible haha.