Only getting back on the forum now! Teachers in dept. have free periods Per.1 (Free periods! what a concept). Anyway the next day after having tried practically every tip and trick that Google could conjur up, I uleashed a nice long string of expleteves, and went for the full nuclear strike. Re-image the the server drive!! A week or so earlier I noticed an error popping up every other day or so stating that "line at (some such No.) is missing", so decided to "terminate' the problem, "terminate with extreme predudice".
Well that should have sorted that. But it didn't, still no DHCP. So then thought I'll temporarily set the Airport Extreme to distribute ip's till I find out the cause.
And wouldn't ye know, it wouldn't accept the static ip addres supplied by Northgate. So that was that for the rest of the day, got caught up in other stuff and didn't get back to it till the following day. Monday morning came in, fired up the server, wondering 'where to now'? Took a look again at the settings in the server admin, opened up the client list, and every machine, every devive on the subnet was there with it's ip listed. Did the Apple Server fairy do some magic over the weekend ? Whatever it was, it's been working ever since.