Hi @CiaranEvans,
it is very interesting that you are ending up with a perfect match. I can not figure out what I am doing wrong. If you have some spare time might be interesting to take a very quick look on the issue http://forum.step.esa.int/t/python-mosaicing-s1-sar-products-problem-s/5138
The picture is not zoomed in but still probably you can see the misalignment with the coast (around 6k i measured it). I am wondering if this is the case cause my Area Of Interest is very north and probably DEM are not much of a help. May be also something stupid I am doing in the parameters as well.
Many thanks for your help 