Slinkylinks,
Fitting a 4-2-1 exhaust manifold (tubular exhaust manifold?) on its own won't give much of an increase in power.
When tuning an engine for performance you have to make sure all appropriate parts are upgraded. For instance, you need to make sure the inlet side of the engine (air filter & carburetor) are able to flow more air and fuel. You also need to make sure that the valves in the cylinder head are capable of handling a higher flow rate. Lastly, you need to make sure the rest of the exhaust system is not restrictive in any way.
You MIGHT experience a very slight gain with just the 4-2-1, but unless everything else is upgraded to suit the gains will be hardly noticeable.
If you have a non FIRE 1100 Uno, you could try sourcing a 1300 twin choke carburetor, fitting a performance air filter and maybe degutting the rest of the exhaust to make it more free flowing. To go even further you could get the cylinder head gasflowed and maybe even have larger valves fitted. That lot with a the 4-2-1 exhaust manifold could well release some useable extra horsepower.
However, if you're going to go that far then really you would need to have the car taken to a rolling road facility to have the engine set up correctly with the new performance parts.
There's nothing stop you fitting the 4-2-1 and then upgrading the other parts as and when. The 1300 engine (of which the 1100 non Fire is simply the same engine with a smaller capacity) has been tuned very successfully in the past. There were a few 128's, 128 3P's and X1/9's with the 1300 engine putting out some very respectable power figures. There's no reason why you couldn't tune your 1100 in the same way.
BUT, if you have the 1100 FIRE engine then your options are very limited. Your best bet then would be to source a 1250 Punto engine, though those only ever came in injected form. And I think all the FIRE engined Uno's fitted with carburetors in the UK only ever had a single choke Weber. Unfortunately, the twin choke carburetor from the 1300 will not fit the FIRE inlet manifold.
Hope this helps, and let us know how you get on.
) yeah, I've got the 1103cc (I think) non-FIRE engine. but it does have a pretty big carb on it (the twin throat webber (might be a 36-38? cant remember)), so I dunno.