The User Guide from the manufacturer of Phostoxin has always said that a full face Respirator with suitable gas cartridge MUST be worn.
The Hazard Data Sheet says that if exposure cannot be prevented through engineering controls, then Personal Protective Equipment including respirator must be worn
The HSE guidance note on “Gassing of Rabbits and Vertebrate Pest–AIS22” makes no mention of respirators but it does other types of PPE such as gloves although it does say read the Data Sheet
Training Instructors are now saying that full face respirators must be worn throughout the whole operation. This appears to make it clear that a suitable respirator must be provided and worn for all gassing operations and staff must be provided with the respirator and made to wear it.
The relevant standard is a full face respirator to EN136 Class 2, fitted with a type B, class P3 combination filter cartridge to EN 14387:2004 should be worn. Arco stock this type of mask and filter ( product codes133300 & 130700)
Other companies also supply them, but the price does not seem to vary much.