If you have an eyepiece you can check the scope for astigmatism by viewing a bright star and racking the focuser in and out. Astigmatism is when the diffraction rings are oval shaped and the orientation of the oval is 90 degrees opposed when inside/outside of focus.
Here is an online
article about optical abberations and how to recognise them.
Refractors can certainly suffer from astigmatism. It depends on the design and quality of the lens. If the elements are misaligned or the retaining ring is overtightened this can cause astigmatism.
The problem you have could be one of alignment. Check the alignment of the camera in the focuser, perhaps a thumbscrew is causing the camera to tilt slightly off axis. Maybe there is an adapter or something that is not concentric with the optical axis.
Bintel are very good and you should certainly discuss it with them over the phone initially and see what they suggest.
Andy