towards end of chapter 2 in the room with long pool in the middle and alcove either side ( where you get swamped by skellingtons on way out). on the right side there is a staff looking switch in alcove halfway up anyone know what its for?

That's the two-way teleporter to/from the sarcophagus in the library. It's very useful if you didn't get the coiled serpent statue, so that you don't have to retrace your steps all the way to the final crypt. A similar one exists in the observatory. You activate it by using Florentine's staff (found in Chapter VI under a bed) on it through the inventory interface. In the library, there are two staff-shaped indentations, one on each side of the coffin. One teleports you to the observatory (the semi-secret room in the basement), the other to the crypt, in case you've forgotten something in either of those two places.