It's helpful to mention what temperature you are using green gas (propane); I imagine it's fairly cool in the UK and I understand gas pistols work well with propane there.
With propane (or any propellant with low boiling points), 40°F will give you very different results compared to 80°F; at very high temperatures you might start to see failures in stock pistols while perfectly fine in cooler regions.
If you're unsure about what propellant to use, I would recommend chrono'ing it. I've found that unmodified Marui pistols are most reliable when it's shooting in the 275fps range using .25