Absent neighbours house in bad state of repair worried it will start to effect ours - what can we do?

you should contact the council in your borough, it depends if they own or rent the house, if they do own then you mite no who they bought it off of and then they will be able to contact the owners.

Bugger. Firstly do you know where the owners are? If so call them and have the conversation you're having here, it could save you a huge chunk of time. Tell them about the problems and see if they react in a way you want them to. If they don't then tell them that they leave you no choice but to get in touch with the council. Call the environmental health up and speak to them. This can be long winded in some areas and enforcement can take a long time. They may speed up if you tell them something like you have seen rats entering the property or people looking for a squat trying to get in. Good luck.

When the 'damp etc' starts to affect your property you should contact your neighbour and ask them to put it right. If they do not take the appropriate action you could either contact the local authority or engage a solicitor to sue them for the damages caused. Prior to the 'damp etc' appearing there is nothing to be done as there will be no genuine reason to bother them.

although it could be illegal, it's just an idea, gain access, let the house out and the money you collect for rent, use to do the repairs on your house, anything left over, you can take me out for a drink for giving such a brilliant idea!

I would suggest you contact your local environmental health and let them deal with it