Nope Bulldog: thanks, but that particular mounting solution won't work for me - not with my TomTom. The only dash-mounted options are either stuck (using the suction mount) to the lower RH corner of the glovebox lid, or else stuck to the glass of the 3-part instrument. Neither are very satisfactory, as I'm sure you'll agree!
I was given a (ludicrously expensive) TomTom mounting kit at Xmas, and used the adhesive clip-in shoe to mount under the dash, in the position shown in the photo. In almost every way it's ideal - the SatNav unit just clicks into place when I want to use it, and unclicks when I want to remove it - leaving the shoe permanently in place (and out of sight). Ideal, apart, that is, from the sound issue.
Think I'm going to attempt a DaveW-type solution, and fashion some sort of sound reflector out of alloy, fibre-board, or sticky-backed-plastic, to go behind the unit and bounce the sound back instead of it being lost in space in the footwells
