Many thanks for your advice Pav. It's just so bizarre that two come off in such quick succession. I've been told that I grind my teeth on occasion, I presume this could have some detrimental effect? Also, is there usually a period of "guarantee" for such things? My dentist has said it's two years.
Your advice is so invaluable to us all by the way, what a great site to stumble upon! If for any reason I have any further trouble I'll know exactly where to look for a different dentist
Thanks again
Anita