Sounds like you`ve bought yourself what some call "a project"

Connection between battery and starter may be good but other connections could be duff.Give them all a good clean.

Treat car to a good service.Personally I`ve removed and cleaned out fuel tanks of this age.Fit fuel filter.Remove air-filter,checking element and blasting carb with cleaner.Clean out the dreaded breather system and change the plugs.Fit some decent new headlamp bulbs.Change oil&filter,cam-belt,fan belt.Spin alternator by hand while belts off to check if rough.Check wheel bearings for noise and play,front c/v boots and rack gaiters for splits,tyres,suspension arms/joints,brake pipes for corrosion.Do your own MOT!These old motors can be lethal if not checked over properly.

Thanks for the replies guys but all that has already been done, as I said before the car is in mint condition, its probably one of the best examples left, its like new all over and has a full service history.

Its a good idea to check the voltage so I'll do that today.

My thinking is that something has gone wrond on the top end so i'll take the top off and see.

Dim lights indicate alternator problem. Check battery voltage and try charging the battery and see if that helps.

Misfire could be that, with engine running, pull off HT leads one at a time. If no change when you remove one, that is likely to be the suspect cylinder. Removing lead from a good cylinder will make an obvious change to engine running.

Wouldn`t have thought leak from crack in exhaust manifold would be a problem although could fail an MOT if bad enough.How old is exhaust system?,Pop underneath and give boxes a shake to see if any loose internals.

Ignition leads break down with age,check for arcing in the dark...or if you`re brave(or stoopid like me :-/ ) and don`t have a bad heart or pace-maker fitted!,then grab hold of them whilst engine`s running.

It might be a minter but fuel breaks down,fuel systems clagg up over time.Good old British weather allows plenty of condensation in the fuel tank and people will run tanks too low.Have you used fuel cleaner?