This depends on the covenant contract drawn up when 'your' property was seperated from the whole. You can check by going to 'The Land Registry' & paying �12 to download the registration plan (�6) & details (�6) of your property. You will find out all sorts of information like who the mortgage is with & the property boundaries as registered. There's also a really neat bit where you can overlay property boundaries on to a satellite photograph! It's amazing how poeple take liberties with boundaries when unchallenged!
Also listed will be the documents that would contain the relavant 'covenant' conditions of access. You then have to apply seperately for these - costs listed on site. I suspect that you are entitled to unimpeded access at all times but you'll either have to pay for the documents or contact the landowner of your property to view a copy of the covenant which is usually attached to the deeds (held by the mortgage company or a solicitor) to get proof. As this is private property the police will not be interested as it is a civil matter unless you are physically or verbally attacked or attacking someone else! That's harrassment, intimidation, breech of the peace, etc..
However, the best approach is usually to talk directly to the users/ abusers of the land. Talk to the other residents & car drivers & explain that you are entitled to (& need) vehicular access. Be careful not to accuse or make them wrong but ask for their help & co-operation. Say 'it's a small world & we all can give & take to get along.' Be prepared to compromise - parking is often at a premium. maybe you only need access at certain times or it's the noise of slamming car doors that is most disturbing? Let us know how you get on.