Take off the heat guard on the back of the alternator and check the connections, if the problem persists then you may need a new alternator.

The rev counter is driven from the alternator so that is why that is playing up too.

Yep, James is right; there is a largeish plastic covered spade connector on the back of the alternator. Check to see if it is loose, clean up the contacts and add a bit of electrolube for good measure. If that doesn't cure it, the problem is probably with your brushes, and I know that they can be changed on some alternators, but not all. If you don't have much technical savvy, call a local independent specialist and ask.

Whereabouts are you? Someone might have a good contact for you.

i am in chesterfield derbyshire