This morning found some yellow patches on the lawn. Further investigation confirmed that this is slime mould.

"This disease does not infect grass tissues. It feeds on bacteria and decaying organic matter. It will not harm turfgrass. As soon as continuous sunny dry weather occurs, the disease will disappear. Light raking or disruption by brushing will hasten its eradication. No fungicidal control is necessary. Hope this info helps."
Thanks, Chuck
See also
http://turfkinghamilton.blogspot.com/2009/07/slime-mold.htmlwww.hamiltonturfking.ca
The Lawn Experts