oil warning buzzerw -PLEASE help!

["Ed Shiers" <mamn97@pop.dial.pipex.com>]

Hi

I've just serviced my car - oil change and oil filter change.
After driving for about half an hour, the oil warning buzzer came on. 
we pulled over, had a look at level. nothing too unusual but the 
level was on the upper limit so I let someoil out of the sump to get 
it reading on halfway. Getting the car back home, the buzzer's still 
coming on at higher than about 2000 rpm, cruising real gently.
got the car back home anyway.Any ideas what the problem is? It sounds 
bad that it only happens once I change the oil - the new oil was same 
as last -10w 40. the filter was a mann filter.
How do I check the buzzer? but this isn't likely is it.

The car does have an oil pressure gauge which doesn't work. Could 
anyone please tell me the wiring from the sender. - there's only one 
wire coming from the sender(the thing above oil filter?). So If I can 
get that working I suppose I'll now that there is real low oil 
pressure.

Any ideas if it's due to poor filter or related to my recent service.

Or (my worst nightmare) is the engine knackered?

help desperately needed

thanks 
ed