Very arty Dave! Is it a Young Frankensun? Obviously not. I haven't heard anything of Frau Brooker (neigh)... oh, there she is.
It looks to me like you're changing your exposure setting between capturing parts of the mosaic. It's vitally important to use the same capture settings for each part of the mosaic so the brightness of features at the edges are the same. I set my exposure for the centre of the disc (the brightest part) then do not adjust anything except maybe focus if I think its drifted.
If you are stitching the mosaic in PS you can cheat a bit if the edges are still showing after stitching but feathering the edges with a black brush in the mask for the offending layers before you flatten the image.
Sad to hear about your Lunt Blocking Filter AGAIN... I can imagine how frustrating that is. I haven't taken mine off my scope to check...
I've had enough technological troubles lately (computers, tractor, car...) ... I don't want to look for more ATM.
Al.