Be patient, but persistent, Andrew. In the big scheme of things, for the person who makes the payment, paying the window cleaner will be low priority. Try and get the number of the person who makes the payment and phone them direct.
In a previous life I was the guy who paid the bills for a large organisation, and I was both overloaded with work and usually hung-over and lazy, so all the 'triv', paying the small bills, didn't really matter to me; I was usually to busy 'fighting metaphorical fires' from stuff I should've done previously.
But I always got round to paying them in the end. I doubt it's anything nefarious that's delaying your payment, it's probably just bad admin.