Well, I snapped it!
I have sent an e-mail to Hornby USA, but there is a big notice on their website: "If your enquiry is regarding parts and spares – please note we do not currently hold stock of any Airfix spare parts." So, I'm not especially hopeful. In the meantime, I have ordered a new Blenheim I, and if I get a spare part, I'll use it to replace the one I raid from the new kit. If not, I'll figure out a more radical WHIF for the new kit that involves a new nose (trimotor Blenheim, maybe?).