After reading all the posts and doing some more research we've decide to buy new (loose fitting) rain jackets.
We just need to find jackets that have the most vantilation flaps like on the armpits, across the back and if possible across the breast.
It seems too much of a hassle to put on a poncho, worries about having to get off the bike quickly without having it hang somewhere, wearing it on stormy days and getting blown off the bike with a strong crosswind gust.
Thanks for all the replies, you've helped us make up our minds.
cheers George and Monique