Thanks Jimbro.
There is a little mark on the distributor cap, like a scratch. It's pretty small. I have changed it for a new one and it still has a misfire. I'll have to check the new one tomorrow.
What causes them to chew the distributor caps? I've noticed that mine is a pain to fit right.
I'd rather it needed a new distributor cap than the manifold gasket. I'm not looking forward to removing the manifold to be honest.

I never found out why my old car did it - the only conclusion I could come to was the distributor was getting hot and expanding slightly, putting the tightly fitted cap under stress. The cap is supposed to be thermally stable plastic but I am not 100% convinced that it wasn't getting slightly brittle after repeated heat cycles.

The manifold isn't too bad to remove from the head as long as you take a bit of care.

The gasket has been changed. It had blown on one side and was generally in poor condition.
The carb-manifold gasket was pretty poor too.
Gaskets changed and the misfire has gone! Thanks for the great advice Jimbro and fellow enthusiasts.
Couldn't change the distributor cap; the motor factors gave me the wrong cap! The car is running a lot better.
The ticking noise I could hear when under load turned out to be the timing. A bit of adjustment and the ticking noise has gone and the throttle response is also much better.
Once again. thanks folks.

ya changed water pump ........

is it belt drive ......... has the timing slipped ... or jumped a tooth ..........

only a thought ..........check and re adjust ,,,,,,,,,,,,

my thoughts