Thanks m_c. Part of me was hoping to be told that I should upgrade straight away but it's good to know that I don't have to spend hundreds more and delay production further.
ExelCNC have said:
We do have customers using fusion360 however they have always set their own PP up. I have attached a PP breakdown and sample g code so you can match an existing PP or get fusion to create one.
And the accompanying info is along the lines of:

You must ensure that you have the correct post-processor within your CAM software setup. If you
are supplied and trained on ucancam we will do this for you...


...If you are using other software then you will need to amend or rewrite a similar post processor. For
vectric g-code arcs (mm) works well. Here is some sample G-code below so you can see the header
and footer required from the A1S.
And then the G-code:

Code:
G90G17G21 
M06 T1 
M03 
S18000 
G00 Z10.000 F12000 
G00 X255.503 Y81.132 Z10.000 
G00 X255.503 Y81.132 Z3.000 
G01 X255.503 Y81.132 Z0.000 F2000 
G01 X255.537 Y81.346 Z-0.058 
G01 X255.682 Y82.316 Z-0.321 
G01 X255.817 Y83.287 Z-0.584 
G01 X255.941 Y84.260 Z-0.846 
G01 X256.056 Y85.234 Z-1.109 
G01 X256.161 Y86.209 Z-1.372 
G01 X256.256 Y87.185 Z-1.635 
G01 X256.342 Y88.162 Z-1.898 
G01 X256.417 Y89.140 Z-2.160 
G01 X256.482 Y90.119 Z-2.423 
G01 X256.537 Y91.098 Z-2.686 
G01 X256.582 Y92.078 Z-2.949 
G01 X256.617 Y93.058 Z-3.212 
G01 X256.642 Y94.039 Z-3.474 
G01 X256.657 Y95.019 Z-3.737 
G01 X256.662 Y96.000 Z-4.000 
G03 X89.861 Y31.168 R-96.000 F5000 
G03 X256.662 Y96.000 R96.000 
G00 X256.662 Y96.000 Z10.000 F12000 
G00 X40.139 Y533.242 Z10.000 
G00 X40.139 Y533.242 Z3.000 
G01 X40.139 Y533.242 Z0.000 F2000 
G01 X40.507 Y533.538 Z-0.127 
G01 X40.890 Y533.858 Z-0.260 
G01 X41.267 Y534.186 Z-0.394 
G01 X41.637 Y534.522 Z-0.528 
G01 X42.000 Y534.865 Z-0.662 
G01 X42.356 Y535.215 Z-0.796 
G01 X42.705 Y535.572 Z-0.929 
G01 X43.046 Y535.937 Z-1.063 
G01 X43.380 Y536.308 Z-1.197 
G01 X43.707 Y536.686 Z-1.331 
G01 X44.026 Y537.070 Z-1.465 
G01 X44.337 Y537.461 Z-1.599 
G01 X44.640 Y537.857 Z-1.732 
G01 X44.936 Y538.260 Z-1.866 
G01 X45.223 Y538.669 Z-2.000 
G03 X48.468 Y545.359 R24.791 F5000 
G03 X49.583 Y552.711 R24.791 
G03 X45.223 Y538.669 R-24.791 
G01 X45.487 Y539.062 Z-2.127 F2000 
G01 X45.758 Y539.481 Z-2.261 
G01 X46.020 Y539.906 Z-2.394 
G01 X46.274 Y540.336 Z-2.528 
G01 X46.518 Y540.771 Z-2.662 
G01 X46.755 Y541.211 Z-2.796 
G01 X46.982 Y541.656 Z-2.930 
G01 X47.200 Y542.105 Z-3.063 
G01 X47.409 Y542.559 Z-3.197 
G01 X47.609 Y543.016 Z-3.331 
G01 X47.799 Y543.478 Z-3.465 
G01 X47.981 Y543.943 Z-3.599 
G01 X48.153 Y544.412 Z-3.732 
G01 X48.315 Y544.884 Z-3.866 
G01 X48.468 Y545.359 Z-4.000 
G03 X49.583 Y552.711 R24.791 F5000 
G03 X48.468 Y545.359 R-24.791 
G01 X48.604 Y545.814 Z-4.127 F2000 
G01 X48.738 Y546.295 Z-4.261 
G01 X48.863 Y546.778 Z-4.395 
G01 X48.977 Y547.264 Z-4.528 
G01 X49.082 Y547.752 Z-4.662 
G01 X49.177 Y548.242 Z-4.796 
G01 X49.262 Y548.734 Z-4.930 
G01 X49.337 Y549.228 Z-5.064 
G01 X49.402 Y549.723 Z-5.197 
G01 X49.457 Y550.219 Z-5.331 
G01 X49.503 Y550.716 Z-5.465 
G01 X49.538 Y551.214 Z-5.599 
G01 X49.563 Y551.713 Z-5.732 
G01 X49.578 Y552.212 Z-5.866 
G01 X49.583 Y552.711 Z-6.000 
G03 X6.508 Y535.969 R-24.791 F5000 
G03 X49.583 Y552.711 R24.791 
G00 X49.583 Y552.711 Z10.000 F12000 
M05 
M30
Which I'm guessing is useful to someone who knows what they are doing
Knowing which post processor I should start with should be a big help.