Just replaced head gasket and head bolt #1 snapped
About 4cm of bolt came out so it's near impossible to get at.

The other bolts are now fitted and torqued up.

Is there any point carrying on or should I just tow it to the scrapyard now ?

Well you have gotten this far, why not buy a new set of headbolts and another gasket and put it opn the road? Once the head is off you should be able to take out the broken piece of the bolt.

The broken bolt is holding the head on firm.

I'm not sure how I would get the head off again.
Or is it worth finishing the job with a broken bolt and hoping !

Well as yuo have gotten this far and the bolt is holding the head on give it a try and see what happens at the end of the day you've done most of the work already at worst you will have a leak and require another headgasket once that happens then you can decide what to do.