The Heron had unsupercharged Gipsy Queen 30's rated at 250 HP, you may be thinking of the Dove Zolt, which had Gipsy Queen 70's, which were supercharged, starting out at 340 and ending with 400 HP.
Oops
You are so right. I did confuse it with the Dove engine.
Put it down to memory slipping with years gone by.
Thanks Brian