PCBs (12 of them!) arrived from dirtypcbs.com, they look great!  A couple of issues to resolve...got the hole sizes wrong for the battery holder so needed to re-drill.  Drilled through a tracing so had to add a jumper.  But after that they work!  Will update eagle files ASAP.  

The main issue now is motor mounts.  I am using the hot glue method at the moment.  But if suitable wheels could be found (see for example ZaidPirwani's comment down below) then we could look at a clip or other method for mounting the motors parallel to the board.