whats the milage mate? going by the fact its a mark 1 im guessing over 60k? take the plugs out have a look might be dirty or dripping with oil due to rocker gasket leakage...if they are oil ridden, then change rocker cover gasket and replace plug...or clean plugs until you can afford new plugs.
Check HT leads as well...make sure they have good contact and are not torn/split.
Also check battery terminals make sure its not rusted, give it a light sanding with wet and dry.
should be enough to get on with