Well I finally got round to cutting it open. There are four plates inside.
The one nearest the manifold holds the trumpet (that's the bit in the middle that you can see in the main picture). In my case, I can see that this is in far from perfect condition but the trumpet is at least being held centrally.
The next plate holds, I'm not sure how to describe this (it's not in the pictures), a sort of muffler, a pipe of about 4" long, 1" diameter, which is crimped at one end (furthest away from the manifold) and has holes drilled in the side.
There are then 2 further plates towards the end. These have 2 pipes in which are offset against each other.
Plans: it's in poor condition so I don't intend spending loads of cash on it. But, I do intend trying to resurrect it. I need to get a plate made to hold the muffler and get that welded back in. I'm thinking of just getting one of the back 2 plates fabricated and welded back in.
I'm sure this will make it noisier and will effect performance in some way.