After action report.
First thanks for the advice guys.
Bad news is I managed to peen the side plate. whacked away at the frame a few times, didn't come free. Repositioned my grip, forgot to put thumb over side plate and gave it another whack. Sideplate popped free and fell onto the cloth pad I was working over. Unfortunately Its now slightly lifted along the edge where the old fifth screw would be. Obviously it pivoted on that point as it fell off. *sigh*
I gave the action a good hosing out with Outers Crud Cutter, then gas, then Rem-Oil as per RC's instructions and put the thing back together correctly.
The toe on the bottom of the mainspring that locks into the cut out in the frame had slipped out. I repositioned this, put the side plate back on and tightened the strain screw down. Trigger return still failed maybe one out of three. So I backed off the strain screw half a turn and tried again. Perfect trigger and hammer action.
I Shot it tonight with CCI & Winchester ammo. 100% ignition and the DA trigger is much smoother - in fact I didn't think to shoot it in single action, I was having fun with the DA.
So two further questions:
a) As I had to back the strain screw off half a turn, should i assume the mainspring is on its way out?
b) The peening on the side plate. Is that an idiot mark I should leave along or would tapping it with a mallet to try and make it flush again be advisable?