just an update - had some unexpectedly cloud free skies this evening so was able to test again. By futzing with the low and high value settings in the solver settings it started solving and eventually landed on a low of about 570ish and a high of about 819ish. I was able to then run through the polar alignment process and complete it although there are a few snags to be wary of;
When you get to the actual alignment phase after the solving and you need to click refresh it defaults to a 1 second refresh and this seems to translate into a 1 second exposure (I imagine this isn't news to everyone more familiar with EKOS than I but it's not clear from the info on the screen screen) , this needs to be set to 0.5 or lower otherwise it crashes (and locks up) as I believe the V4L driver doesn't support beyond an exposure length beyond 0.5s. The other snag is that the image you use to align the star into the crosshairs has a _lot_ of noise on it (no idea if this is expected behaviour or not but the noise looks like what you see in the dark frame). It is, however, just about possible to make out the point you have selected although getting it super precise wouldn't be that easy.
In summary I had to ensure the following was set to make everything work;
Ensure in the indi control panel the iPolars image size is set to 1280x960 otherwise you will get strange images indeed.
Change the gain in the iPolars settings to a level suitable for your skies. This was in my back garden which is bortle 6/7 so I found I needed the gain at at least 160 to get solving to happen and right up at 192-288 to get it happening reliably.
I set the FOV in the alignment module to 780' x 585' and this seems to be happy.
I set the low and high values in the solver settings initially to around 400-900 and then it eventually homed in on about 571-819 after it solved the first image.
I used an exposure of 0.25s.
I used a refresh of 0.5 on the final screen.
With that in place it's then working for PA, with the noise issue mentioned above.
So I've successfully used it for alignment and I hope the info helps anyone else wanting to do the same.