Barry,A properly fitting English saddle does not need a pad of any sort.
Years ago cloth saddles came into use in order to help keep the saddle clean - but the use of a saddle "pad" to help an ill fitting saddle to fit is not a good solution.
Sweat under the saddle would be minimal if there was no pad. The gullet is designed to provide a good flow of air.
If I clip out my horse during the late summer or the spring to help with the sweating problem, then I leave a saddle patch of unclipped hair to protect the horse's back from abrasion.
If you do fit a pad or cloth, then it should be of a natural fibre which can be washed regularly.
I also find that modern "coolers" which have a very effective moisture absorbing fabric built in are excellent for promoting the sweaty horse to cool down after exercise. I always avoid leaving a hot sweaty horse in a cold breeze.
Some new saddle pads are said to be designed to absorb sweat and to be washed but be careful - any materials other than cotton or natural sheep's wool are suspect until proven otherwise.