first, download the mk1 manual available on the site.
i encountered a similar problem with an sl 1600 mk1 i purchased recently. the cueing platform wouldn't lower even though the lever was moving up and down freely. what i found was that the inner mechanisms of the table were stuck mid-cycle. basically, in auto-mode, the START lever activated a series of gears and movements that once started need to move through their rotations. usually, moving to the STOP position is enough to cancel and disengage the table, but if the table is older and hasn't been used in a while it's possible that the sliding mechanisms have gotten stuck. you can either rotate the platter manually until you feel a click (let the tone arm float and you'll notice that it will suddenly move and return to the rest position) or you can take the table apart and lube it up. either way, if the lever moves but the platform doesn't lower, something is holding that piston up.
to take everything apart is pretty simple; take off the headshell, counter weight, and be sure to lock down the tone arm with a twisty tie. then remove the platter and remove the metal plate underneath. once you got all of that, there are three connections you need to disconnect the table from: the main power (8 or 9 pin connector at the front of the spindle mounting) the board power connection (2 pin with a brown wire that runs under the main power connection location. the connection to the board is on the upper left of the openning) and the tone arm connection (5 pins that connect a small circuit board to the wires running out from the tone arm assembly). just be careful as you disconnect everything and there may be some old tie downs you need to cut.
once those are disconnected, flip the table and take off the feet. i usually put the table on its side to prevent any damage to the arm, but you can flip it all the way over onto a heap of pillows, blankets, or a beanbag or something. BUT, before you lift the top off, be sure to lay the table down (like normal), release the tonearm, and let it move freely. as you lift the top, you may have to move the arm around to get it through the openings.
with the lid off you can see most of the moving parts. the trouble for me was with the sliding metal plate that runs from behind the spindle to the activation levers to the mid-right side of the table; the grease had gotten hard and it needed some lube. this metal plate is also connected to two gears that are attached to the spindle underneath the base of the table. you can get to the gears by taking off the cover plate on the very bottom of the table. mine was full of dead spiders. awesome.
anyway, after adding some grease and moving the plate manually (by rotating the spindle or the gears under the spindle) i got everything moving again and the cueing platform lowered as it should. this, however, may awaken other problems that you either didn't have or didn't notice. not to worry, tinkering will eventually pay off.
to replace the tonearm lockdown, you'll have to remove the three screws that hold the tonearm assembly onto the base (one screw is covers by the grey removable cap near the anti-skating). once those are out you can remove the entire tone arm assembly and flip it over to give everything a look. i greased up the cueing piston, sprayed the springs and got everything nice and flexible. you should also find a screw that is under the position of the lockdown. remove that and you can take off the broken one and replace it. you should be able to use a lockdown for an SL 1200 MK2 with no problems. check ebay.
dust covers are the same: the mounting screw locations for the female hinge brackets at the rear of the table are in the same position as the SL-1200 MK2. just get a sl 1200 mk2 dust cover (usually $35 on ebay) and a set of hinges (the male dust cover mountings AND the female brackets that mount onto the actual table - should be another $15 or so for a full four piece set) and put that sucker on.
remember, while you've got the thing apart, you may as well spray the tuning pots. i used CVC 5-56 (it's a good lube that works with plastics)but most folks perfer DEOXIT (lube and electrical cleaner). there are four pots in total; the two larger pots mounted under the grey top plate connected to the tuning knobs on the face of the table and two pots on the circuit board that offer a wider tuning range. give these all a pretty heaping helping of spray, work them around and repeat until they move easily.
put everything back together, and give it a test. remember, to pin down the speeds you may have to adjust all four of the pots. you may have to take the table apart several times to get a real feel for how all the parts move and work together. but you should be able to get this all finished up in a few hours.
also-there's a user by the handle BRICK that has several posts about disassembly, tuning, cleaning, etc and lots of great pics.