Bugsy, you're 90 percent there. Just add the first name of your washboard player and you become:
Washboard Sally and the Sandgropers Skiffle Band.
Come to think of it, call your band that even if your washboard player is John or Bob. When people ask, you can blame it on an age-related memory lapse.
OR: Call yourselves the Wiggles. That'll draw a crowd. Once.
Let us know what you decide. Also, this process might make a good story for your local newspaper. I bet you're getting suggestions from all over the world on Mudcat. I, for instance, am in Detroit.