The feedback I received from the first agent was very positive and specific enough for me to make some changes to the first five chapters including the addition of new chapters which revealed the backstory of the protagonist I more detail.
Sending the book to another agent meant first finding an agent who represents an author I like whose work reflects st own in some way. Having identified an agent I could then go to their web site and find out their requirements for submissions. This particular agent asks for the first three chapters or 50 pages. Given that my chapters are relatively short, I opted to send the first 50 pages.
Sending material to a new agent means rewriting the synopsis and cover letter taking care to explain why I have chosen to send my material to them.
It’s all done and been sent now so while I wait three months for an answer I’ve got no excuse but to get on with writing the next novel.