Hi Alistair,
Here is an example of a good night at my site, FWHM ~2":
http://www.rolfolsenastrophotography...g/i-bNVbbNZ/X3
That is measured with PixInsight. I get similar results in Maxim for the same frame, so expect it to be reasonably accurate.
First of all I'd suspect that your image scale is set incorrectly so FWHM is not reported properly, but if you say images are also blurred compared to others then maybe not. Just something to double-check.
And secondly, collimation could be off.
I don't know if your site has particularly poor seeing. If it's always 4 or above then that sounds odd, but I'm really just guessing
What is the experience of others in your area?
Cheers,
Rolf