The jam night is more of an open mic night really. The guy who runs it is generally happy to let you use his bass, so you wouldn't need to bring one, but they tend to have prepared acts on, rather than ad hoc/on the night combinations.
Closer to where you're staying, [url="http://www.wharfchambers.org/event/the-benniesbear-tradethrowing-stuffselmer-thurmathe-no-marks/"]Wharf Chambers [/url]has a punk gig on the Monday night