Haha, Agatha Raisin is hilarious. I started her books earlier this year and after reading one, I then download the next onto kindle and so on. Well worth the read.
My favourite of all books are Brother Cadfael. If you want to go down the medieval route, there is a great series of books by Paul Doherty called the Sorrowful Mysteries of Brother Athelstan which take place in Southwark in South London. His descriptions of London are fantastic (Southwark being more like a village than a suburb in those days). There is also a series of books centred in Exeter by Bernard Knight called Crowner John which are worth exploring.
Great set of books these. I read them a while back when I was visiting the Lake District. I believe there are 4 in all.
There is a good series of books set between the two wars about Maisie Dobbs who is a private investigator. Although she is based in London, her work often takes her out to the Kent countryside (based not too far from my home actually). Very enjoyable series too. The write, Jacqueline Winspear is a Kent girl who now lives in US.
The Inspector Morse series are great too, Katrina, set in the town of Oxford, by Colin Dexter.