My 1200 ute has an a15. I have tried to time it with a timing light but the mark is moving between about 18 and 22 degrees BTDC at idle. I have replaced timing chain, points, leads, coil, ballast resistor, distributor cap.
What could cause this movement?
I also had the distributor springs installed at a shop to take that out of the equation.
Please help this is doing my head in(and wallet)

that is a symptom of your dizzy start your advance curve at idle speed. usually this means weak dizzy spring(s). an idle too high can also make this happen. the shop should set the advance curve higher than idle speed. if you can, try lowering your idle and see what happens. if same situation still happens, well, you need stiffer spring(s).

You have replaced everything that normally causes it:
* Points & condensor
* slack timing chain tensioner
* broken timing springs

Now find out the cause:
* Double-check the timing dwell
* Adjust idle mixture and idle speed
* Check Tensioner for proper installation

Thanks guys, will check timing chain tensioner. I think it is worn(deep groove in the chain side)
Also, would a weak spark from coil make the timing mark move?
The coil is producing spark but it is very weak. I rewired the coil and ballast resistor as the wiring diagram showed. Before I did that they were both getting hot.

It could also be a worn distributor bush causing the erratic timing.

You're also way too far advanced. At idle you should be about 7 or 8deg BTDC.

Good idea, check the shaft bushing. You don't need to remove the distributor, just see if the shaft will move side to side any noticeable amount. If so, replace the distributor.

At 20 degrees initial timing, you'll lose power in the top end at best and damage the engine at worst. That's with a normally spec'd A-series distributor. With custom re-curved distributor it may be different.

Yes, weak spark can cause erratic timing. Or more correctly, what is causing weak spark can cause erratic timing (worn bushing, incorrect points gap, etc).

Okay. I replaced the timing tensioner. It started and drove the other day. Went to fill up with fuel and it didn't make it home. I noticed the coil was quite hot. Is it likely that bad earth or wires could cause this and in turn making the weak spark?
I am setting the initial timing to 7 degrees btdc but on the light it reads around 18-20. Is this normal? The revs are up around 1000 rpm at idle. I thought vac advance only kicks in after that. Also the mechanic said I could set initial at 12 degrees.
I think it's gremlins. Might have to take it to auto electrician I guess

Did you replace the coil/points ect because it wasn't running right or because of the timing mark moving?because if your setting the initial timing staticaly 7*btdc but the light show's 20*advance it would be pinging its head off,so it could be a crook timing light,but if it was running bad before you replaced everything i'd suspect somthing mechanical,check the dissy drive gears on the dissy&cam for any freeplay,as for a weak spark remove the ballast resistor and see if improves the spark you may have got a non resistor coil