Just a guess: white plectranthus?
Rusflorum: I think you've got it! Fantastic - thanks so much
That's a new one on me - I'll keep my eyes open for it
Actually I think it's a Plectranthus hybrid called 'Mona Lavender' hence my Dad thinking it was part of the lavender family. In fact they are related to mint and deadnettle. Good website here: http://www.plantzafrica.com/plantnop/plectranmonlav.htm
Good on you rusflorum. that plant had been bugging me. I was convinced it wasn't a Salvia , the leaves didn't look right.
Look ideal for a cool conservatory
Thanks. The local supermarket is always selling loads of these, they flower
for ages and are trouble-free.