G'day Andy,
Congrats on the new mog. Hope you get heaps of enjoyment out if it.
The cylinder in question could well be a fuel catalyst. The makers claim they eliminate the need for an additive when using unleaded petrol in a vehicle without hardened exhaust valve seats (eg standard Minor).
They often make many other pretty amazing claims relating to fuel economy and power.
My understanding of these type of catalysts is that they are not effective at preventing valve seat recession, but otherwise have little or no beneficial effect. I stand to be corrected.
Vast and extensive volumes have been written and arguments had by bigger minds than mine on the whole unleaded/hardened seats/valve seat recession thing.
Let me state clearly that I'M NOT AN EXPERT on this matter, but have tried to read as much as possible on the issue as leaded petrol has been eliminated in Australia as well. Nothing I have read outside a manufacturer's literature provides clear evidence that these things work. Practical Classics magazine withdrew catalyst manufacturer's advertisments after independent tests failed to show the stated results.
I suggest using an additive (eg Valvemaster, Flashlube etc), or getting hardened valve seats fitted.
This is the website for one of the catalyst manufacturers:
http://www.broquet.co.uk/
Cheers!
s
there is a metal cylinder approx 6 or 7" long and 1 or 2" in diameter connected via tubes between fuel pump and carb, previous owner said it was an unleaded conversion which i took with a pinch of salt being the worldly wise astute person that i am 
could this be the very thing ? and if so what does it do ?
Probably wasn't doing much, then... It's still in the tank actually, but I've had a new cylinder head, which is the best way of solving the U/L problem. 