keramos Is there a relation between Polar alignment error (as seen at the end of polar alignment in ASIAIR) and guiding error (Total RMS)?
Yes.
If your mounts polar axis is perfectly aligned to either the NCP or SCP (will never happen in practice :-) and if you mount's two axes are perfectly orthogonal to one another (in practice, there is also a small error, depending on how well the mount is manufactured and QA'ed) there will be identically zero error in declination.
In practice, auto guiding will take care of it.
Except for getting rid of field rotation.
If the polar axis is not perfectly aligned to the NCP or SGP (you mentioned UK, so it will be NCP for you :-) you will find that the sky will rotate around the star that you have chosen as a guide star. I.e., the guide star will track perfectly, but other stars will rotate around it.
If you are polar aligned to within 1 arc minute, the rotation is small, and if you keep the exposure times to less than 300 seconds, the stars will still be round across the plate. A plate taken a couple of hours later, however, will show that the star positions have rotated, but pretty much all stacking programs nowadays will de-rotate the plates for you.
In short, don't be obsessed by polar alignment error. 20 arc second to 30 arc second is plenty good enough. Trying for better is just wasting precious time (unless you are experimenting and evaluating ASIAIR's Polar Alignment accuracy :-) :-).
There are calculator programs on the web that can figure out how much a field has rotated given the polar error and how long you have exposed. Just look for keywords like "field rotation" and "calculator" in your web searches. Together with plate scales and the FWHM of stars on your plate, you can figure out how much polar alignment error you can tolerate (I'll bet it is more than 2 arc minutes :-).
Chen