I would refuse to do it, or at least offer quarterly visits with visual inspection only. In the event of activity being found (inevitable if they have ducks) then I would charge per visit until any infestation is cleared up.
My idea of pricing would be:
Quarterly visits to inspect, recommend and report accordingly £180 p/a
Initial visit to treat rodent activity £60
Follow up visits (twice a week) £30 each
The alternative is to do a more traditional type of contract with monthly visits to inspect, check traps, check non toxic bait points, recommend and report £720 p/a with all fo