bubba
a few things re: neighbours:
do they have planning permission for the foundations? contact building regs dept at local authority for info. did they publicise it (if they didn't it's against the law anyway).
it is a civil, rather than a criminal matter, however the police can forcibly remove them from your property, especially if they are damaging it, or trespassing.
the council MUST have given them some form of permission for such major building works (unless this is foundations for a shed or something v. little), therefore there is someone at the local authority who will have been dealing with their applications/permissions and work arrangements.
see above and contact via planning and building regs people.
you can also report them for failing to submit appropriate planning applications etc to the local authority dept which should stop the work.
yes, you may eventually need a solicitor, but in the first instance - they cannot work on your property without your consent, which you have not given and are denying.
all this based on the assumption that you own the property and the neighbours own theirs and that neither are social housing or rented....
HTH, but let me know/PM if you want any other info. I used to work in property services in councils and have dealt with planning before, so if you need any info, give me a shout. x