5k training routines?

I run a weekly 5k race on Saturday mornings (Milton Keynes PARKRUN), mainly for fun but also trying to improve on my PB. I peaked last year and have been going backward ever since. I managed 22min23sec at best but have slipped and can't get within a min of that time now. I run twice a week in addition to the 5k runs, usually for about 6-8 miles each. I was wondering if there was a training program I could follow to try get my 5k times back down to where they were or even better. Thanks in advance. Mike,