Thanks for the tip Frank, down in the crawlspace, we still have all our son's toy boxes. There's a big box of Lego down there so I might give that a go.
The mould has cured and I've extracted it out of my mould box, and got the master part out too without any problems. Looking as best I can inside the mould, it looks pretty good, can't see any bubbles (something I was warned about) and the relatively simple detail to the air intake seems to be OK. I am now going to try to cast a replica ----