A friend of mine who is a little more technically proficient than me confirms that it's a fairly simple bodge to get the race blades to fit. More a question of measure twice and cut once as opposed to anything else.
Having spent the evening reading up about saddles and saddle fit, and having measured my bum via the tin foil & carpet method, I've come to the conclusion that the supplied junior saddle is probably slightly too narrow. Will measure it up and compare with the saddle on my old bike - a cheap no-name thing that's seen better days. I always seemed to get on ok with that one. Could always swap it over and see if it helps...
If it does, I can always look for something similar. If not, back to the drawing board I guess...