Evening Graeme,
I've searched pretty hard for your elusive amphibian during work breaks today, but I have to agree with
one11, it does look like the Mikhelson/Nikitin Mu-4 trainer flying boat.
This is all I could turn up, but it seems to fit the bill.
MU-4 and MU-5 flying boats by N.G.Mikhelson/V.V.Nikitin
I'd say well done
one11 