Loved the Courtneys and the Ballantynes and have read most of his stand alone novels.
When he brought out the Egyptian ones ,River God etc .I thought ,great, but they were awful and I've never read anything by him since .
He is getting on now ,must be over 80 so it wouldn't surprise me that his latest are co written .In fact from a review on Amazon in 2013 it says ...
Wilbur has just signed a £15 million deal for 6 more books. He supplies the story line and "selected" writers actually put pen to paper. His previous publishers of 45 years standing, Pan McMillan, didn't like the idea so he has now gone to Harper Collins.