Dave,
Spoke to Williamson's.
To cut a long story short, Shurflo will not design a product specific to the window cleaning industry needs as we are just a very small portion of their sales. The current pump has just been slightly modified to meet our needs.
The guarantee will only cover you if there is a design fault, not if the fault has occured during use because the pump is not designed to pump water through di vessels and microbore as the pressure is too great.
Doug