This adds to what rabbity girl says, which is basically correct. This is nothing to do with 'plans', but is to do with the Title Plan held at the Land Registry, which is what counts (Deeds are only useful on land not registered at the LR).
The Title Plan, which you can get a copy of online at the LR for a few pounds will show the boundary line, but maybe not in enough details to show the exact line, unless there is a fence there.
The neighbour has no legal right to come onto the land during the course of constructing foundations (without prior agreement - see below) and NEVER has any legal right to put foundations on your land.
The Party Wall Act covers situations where a neighbour wishes to build right up to the boundary line. See the link below and read about the requirements of a developer. The PWA does then enable an owner to come onto the land DURING the construction (to put scaffolding up, say) but any damage must be put right. Beware if the neighbour thinks they are going to build a wall right up to the line - how are they go deal with rainwater guttering and roof overhang without hanging over your side of the boundary? - they are not permitted to do this and should be building back from the boundary line to allow for it. As you have also indicated above, building a wall on the boundary is really difficult without the foundations straying over the other side and they are not permitted to do it.
Once you have read the stuff on the Party Wall Act I would confront the neighbour with it. It is their responsibility, not yours to initiate this.
The best way to get the job stopped (without involving a solicitor, injunctions and money) may be to try and involve Building Control at the local council. But you should get it stopped now and resolved before they start putting walls up.
http://www.communities.gov.uk/index.asp?id=113