Randomly knocking on farmers doors is a waste of time.
You need to see somewhere that appears to have a problem, and then find the farmer and offer to fix that problem.
For example : there's a load of pregnant ewes, but there's lots of crows and jackdaws too - those will take the eyes out of new-born lambs. You can offer to fix that problem. Which saves the farmer money.
In summer, a farmer's wheat crop is getting hammered by pigeons. Problem, you offer a solution.
Bear in mind that a lot of tenanted farms don't have the shooting rights (game shooting), but they do have the right to pest control. Ask too much and you're more likely to get nothing.
Once you've got a foot in the door, keep your promises. Fix those 'problems'. Doing that will get your name known, and will more than likely lead to that farmer's mates offering you pest control.
These days, unless it's a specific 'problem fox', I don't take on any new land. I concentrate on keeping the pest control work done on the land that I've got - 3 farms, 500 acres.