Went well in the end.
A fellow bass player was asked last minute to do the sound (as he has a PA big enough for the event). We heard everything, probably the best onstage sound I've had in a long time.
It didn't rain...thank goodness.
The stage was small so the part set up kit was built around the drummer. We got on really quick, didn't bother with a proper soundcheck...sound guy did set levels for the kit, however, we started our opening song and as all seemed well we didn't stop. Had a good response from the audience....overall, very happy.
EDIT: forgot to add, I didn't take the new bass, just in case of mishap