The riser legs are obstructed by pipework that runs along one wall under the tray. What can I do?

Some companies offer an offset leg adapter, such as Mira (part no 2.1570.557.WH).  

This allows the adjustable legs to be installed offset to the tray, where the pipework runs.

These adapters can increase the gap between wall and leg from 20mm to 85mm, please to check the product details before purchase to ensure they would be compatible with your legs/tray.