Ask an Property Solicitor. Get an Answer ASAP.
If they can not talk to the neighbours and provide them with a copy of the deeds which confirm the vehicular access then they really only have two real options. A solicitors letter would normally do the trick and should be tried - advising them that you would seek an injunction and damages should they not agree to resolving the issue. The second option is the actual injunction application to the court. In a case like this you could get into court very quickly. You could in theory also contact the police and blame obstruction - it will depend on your police force as to whether they take it seriously. House insurance may cover the legal costs. I hope that this helps and happy to discuss. Please rate positive.