Hi all, Many thanks for your replies - i appreciate any thoughts and suggestions.
The vet assures me she's been checked for everything!! Its so baffling. The latest is that a second opinion has suggested a nasal biopsy and an MRI if the drugs don't work. We're probably going to go for the MRI - pricey, but will hopefully get to the bottom of it.
She's had lung xrays and been told they are 'perfect' and the vets have even taken a copy for their own records to use at presentations.
....as for antibiotics, she was started on marbocyl then changed to ronoxan, then to synulox. No improvement with these drugs. Shes now back on the marbocyl, along with another antibiotic called antirobe. She on metacam still and now a mucolytic. poor thing...
we're back to the vets tomorrow, so we'll see what happens....

thanks again all.