Spending 15 years in an "imperfect, but far from terrible" marriage, isn't meeting your needs or making you happy. This is what needs addressing, not the potential affair.
In resisting your urge to cheat and writing this question, you're actively acknowledging your dissatisfaction. Bring the conversation to your "lovely wife." You're entirely lucky to feel she is lovely after 15 years of marriage, many people do not even have that advantage, my friend.
Begin the conversation by figuring out where the source of your marital strain lies. That is what needs to be the focus of further discussions. Perhaps seeking the counsel of a professional therapist would help to serve you best as you and your wife explore the missing elements needed to repair your marriage.
In closing, a simple but key point; kindly and gently avoid this work colleague as much as possible right now. There's no reason to bring great temptation to your doorstep!
You don't need to have lunch, drinks or dinners with her. Try spending only necessary "work" time with her, as you go through this troubling period with your wife. If your colleague begins a too-personal line of questioning as to why she sees you less, or as to why you're being less social, you may choose not to answer her at all. She will get your message very clearly and quickly!. This could help to improve some of your environmental stressors/conditions immediately.
Thanks for reaching out and keep me apprised of your "repair work!"
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Have a profitable and peaceful week,