"The statutory body for investigating archaeological discoveries in Bolton is the County Archaeology Unit, which is unusually administrated by the University of Manchester. Not only do they refuse to go out to look at the new sites they also forbid me to notify them of them - they have threatened to put their lawyers onto me if I do."

So are they saying they have a statutory duty to investigate, and sending them duds is wasting official time? Sort of "Vexatious Notification"?!

Excellent!

Not sure there's an actionable cause there, even if you were doing it maliciously. But as you're doing it in good faith there's not a snowball's chance in hell a lawyer would advise them they have a case.

Apart from which, their Case would require an archaeologist to stand up and say "this is NOT a prehistoric site". As to that happening, see the Silbaby discussion ;)

Send 'em some more. Send 'em a dozen. If they spend public funds on a lawyer send the whole file to the ombudsman citing maladministration.