Acted on advice - removed fuel pressure sensor, (attached hose full of fuel) washed/squireted as per Roger technique.
Reinstalled in slightly higher position (limited firewall height in a Sonex) and possibly more importantly have sensor vertical against firewall.
1.5hrs flight - fuel pressures AT ALL TIMES GOOD!!! - this includes when switching off Boost pump - much less drop.
My heartfelt thanks to all advisors.
