Our Camellia regularly produces many many buds but these either fall off before developing into flowers or within a few days of opening. Conditions - semi-shade, damp soil, some clay. Any advice would be appreciated.
Well Vernon I'd guess that your Camellia is suffering from wet feet.
You describe the soil as damp with some clay which could indicate that it is not well drained enough.
Camellias demand well drained (albeit moist) soil or they can develop root rot (phytophthora) which if too serious will make the plant keel over and die but if not too drastic will just cause it to drop its buds before going to all the effort of opening them up.
I hope this helps - try some gypsum on the soil to help break up the clay and therefore hopefully improve the drainage. Then apply a good layer of mulch which hopefully the worms will drag down into the soil to open it up a bit.
Alternatively you could try an application of Fongarid to the soil which would help treat the roots.