"The espresso doesnt start to pour until about 10 seconds after flicking the switch - is this normal? or is it another telltale sign of overdosing/overtamping? " Umm yes it is, overdosing. I imaging it wouldnt have been much more than a tricle when it did flow.
With a firm tamp, and it can be done with the rediculous plastic thing youve got, you want the packed puck to be up around the groove in the basket. If youve got beans to spare, do a couple of trial fits.
Fill until the basket is full, tap to settle grinds and level off, tamp and ideally you should be at the groove. Pat the shower screen dry. Fit it to the group head, then take it off, without pulling a shot. Have a look at the top of the puck. You should be able to see the imprint of the shower screen screw in the centre. If the Shower screen is wet, some of the top layer of the puck will stick, hence the requirement to pat it dry for this practice.
There shouldnt be anymore than one hand required to set the PF in place. If you need to force it, youve over filled the basket. ;)
Then the rest of the variable come in to place. Fresh beans, grind, humidity etc..
Good luck :)