You can upgrade the exhaust system with either a good set of LCBs (1 3/4 for an 1100) and a RC40 muffler to provide free flowing exhaust. This will increase your power and economy.
If you want to go further to tuning for economy and you are doing head work if I remember correctly fitting larger exhaust valves in relation to the inlet valves will give you better fuel economy. A general port and polish will also help, some inlet manifold work, a better flowing air filter. Raising the compression ratio can also help.
A lot of the modifications that increase power can also increase fuel economy if you don't really demand the extra power your engine is creating (but who does that

).
So economy comes down to driver behaviour as much as anything.
Get Vizard's yellow book it has a section(s) on tuning for economy, but to start with some of his best mods were extractors and muffler, and less restrictive air filter and perhaps inlet manifold. Also if I remember correctly SU carbs are near the best for fuel economy. A dyno tune will also be able to set your car up so that it is running at peak efficientcy, a carb running rich or lean will really knock your fuel economy.