Hawk Classics

If you've jump started it and it shows a healthy charge then the alternator etc. are all working well; the clicking sound is almost certainly going to be the low battery. Your battery might not be able to hold a charge if it is fairly old, however it could simply be that it needs a good charge. Have you tried leaving the battery on a reconditioner for a few days, it might revive it and it's a cheaper option than a new battery in the first instance?

The Mk2 was a positive earth, however, converting it to a negative earth with an alternator is a common change/upgrade, so yours could be either positive or negative. Do you have an alternator or the original dynamo, or al alternator on the right hand side (as you look in from the front of the car)?