And then she really got up my nose and in the end I called her a name I shouldn't have, so that's definitely the end of my relationship with this particular vendor.
Meantime, I've emailed a couple of other petshops and tried to be more diplomatic about how I approach the issue - see, I can learn from my mistakes! I also want to find out if they have any other advice for looking after the mussels than what we've gathered here, or found on forums around the place.
Is JBL NovoTom and some home-cultured green water the best we can do?
Speaking of, I've been using it for a couple of days now, and the little powder flecks of NovoTom do suspend in the water and not get sucked immediately into the filter. It's hard to tell if the mussels are ok but so far they're not trying to escape (apparently that's a good sign?) apart from one of them rotating itself 180 degrees on the first day, and I'm pretty sure they're the right way up (any advice for how to make sure I haven't got them upside down?) and they close up if I touch them gently so I know they're alive.
One of the animates guys suggested sitting them on top of an upturned plate so they'll move down the slope towards the gravel and orient themselves on the way. Would that work or should I just trust that they're the right way up now if they're not struggling?