After a few hours of digging and taking out the filter caps to redo them i discover the cause of at least some of my problems. The 1 megohm pot for spot shape went completely round the bend at some point.
Whether it was pre-existing or the result of a screw up i don't know but i had one rattling round my junk bin, right on the edge of -10% tolerance but all that it's for is a voltage divider, so i've decided to try it.
EDIT: OK I've installed the new pot and triple checked the filter caps. Now i get a nice sharp trace on the screen and almost all is right with it that i can see EXCEPT i still got some popping from the capacitor area.
Now i'm sure they are in right i spent 6 hours just on them, the first pop was so loud i though a cap blew, none did.
The second pop was actually a set of them that happened when i touched the spot adjust, but only the first time.
I was then able to adjust it and test the scope up to 2.5MHz (the highest my function generator goes) without issue.
Maybe the popping was just the sound of the dust bunnies committing suicide