What does this flat tell me about my scope?

The last image of your (M33) shows significant coma so collimation was an issue then. If you were adjusting the secondary then you might well be clipping the light cone but I can't go further than this without looking at a (very) out-of-fucus image of a stellar field.

By the look of it, it seems mildly tilted left bottom corner <=> right upper corner, which is a sad occurence with fast newts. If you can't correct the tilt then the best you can do is to focus so that you get the best possible compromise. This said, however, you might still be clipping your light cone, see upper side light drop off.

Looks like your image is not centered on your sensor.

Lasers are useless for collimating fast newts. The gold standard is a catseye cheshire and autocollimator, which is what I've been using for my 250 F4 Quattro since I bought it. The focuser also needs to be very robust and a threaded connection is preferred. For a couple of years I have a 2" compression ring that holds onto the coma corrector but once I went to a 3" feather touch focuser with a threaded connection, I no longer had any tilt. I also noticed a lack of any pinching, which I put down to the coma corrector no longer functioning as a mechanical support like it did in a 2" compression ring.

Piers Palmer:
Thanks - very useful! They're not readily available in the UK from what I can see, but I could easily get one sent straight from the States. 

The trouble is (and this is probably the tipping point for many people)...can I face it? The focuser is pretty basic and has a bit of play in it. It's got a 60mm female thread on the end of the draw tube so not much connects to that, so it would be sensible to upgrade the focuser , but that's more expense and effort. Also, I've just gone from a 11mm sensor on a 533MC pro to an APS-C sensor on a 2600MC Pro. That will probably exceed the capability of my Vixen Coma Corrector 3 and call for an expensive Corrector ph...etc etc.

If you can get adapters then the 60mm thread would still be useful. My focuser has an M68 end cap, which my coma corrector screws into.
Yes I completely understand the cost pressures on this hobby. If one wants to use fast Newts then the scopes, and it's components, need to be able to accommodate the tight tolerances or it becomes a world of frustration. A sturdy focuser with a threaded connection will go a long way and will last you a long time (doesn't have to be feathertouch but that's what I have and they are brilliant). I have no experience with the Vixen CC so can't say how good it is with APS-C sensors. My first CC was the Skywatcher Aplanatic CC, which I used for about 2 years. It can do APS-C but the vignetting is crazy (~40%). Now I have the Televue Paracorr II and I'll never look back! Round stars in every corner and minimal vignetting (~10%).

Rotational issue of the secondary mirror. I have it also.  The problem is, most new Newtonians do not have a rotating secondary mirror, which may fix one issue (many people ruin their rotation, because lasers don't correct for it) , but ruins another, which is the factory setting is not perfectly aligned rotation. 
You can try to correct it by doing some DIY stuff (check youtube for Newtonian tuning) . I attempted it, but even with a better rotation, I started to have another issue, despite perfect collimation. My stars were just out of shape, almost diamond shaped. 
I returned everything to factory and I was back with the vignette visible , similar to yours, but at least my stars were round again.