leak from thermostat housing / elbow

Hi folks, just doing a coolant change Gerty as I need to replace the coolant flange on the main block. Whilst I've replaced the flame without issue whilst refilling the coolant I noticed a leaking from where the thermostat and elbow connector join. The o ring that's supposed to make the seal didn't seem in bad shape and want leaking prior to be removing it or the thermostat.my question is do I need a paper gasket as well as the o ring where the two meet as well? or should just a new o ring work. The thermostat was put back in first and the oring placed on the elbow as I refitted it so assumed it would seal without issue. Any thoughts welcome. She's  89 gti cabby. Also one of the 10mm bolts sheared during refit so was just going to replace with a standard 10mm with hex head Bolt of same length. I'm assuming that should nt be an issue.

That join is a pain the plastic housings warp so easy best to get a new one if so but don't buy pattern in my eyes I did and still leaked get a straight used one with o ring or try using some rtv silicone on the joining flat surfaces as well as the I ring and yes a noal bolt will work 

Replace the rubber O ring for a new one, always best to fit a new one when ever taking them off.
25p from euro car parts at the moment, might be worth getting 2 but you do get a new one if you buy a new thermostat.
If you did not check the thermostat while it was out of the car might be worth dropping it in some boiled water to check it opens and closes once removed but if you think it's OK and it stays in place once you have took the elbow off, leave it in place as even more coolant will be lost if you remove it again.

http://www.eurocarparts.com/ecp/c/Volkswagen_Golf+Cabriolet_1.8_1988/p/car-parts/car-cooling-parts-and-car-heating/cooling/thermostat-housings-gasket/?369440050&1&4d6daaee3806cb28a922beee18a62e87a5b3d865&000574

Make sure the two faces are clean (elbow and the water pump) and free from any built up crust etc and they should seal and yes any old bolt will be fine as it's not done up super tight.
 

Thanks guys help is much appreciated, I've bought another elbow (therm cover as it described) and this comes with a new o-ring. I did clean the face of the pump \ therm end origonally, but having looked at the old elbow again comparing the new and old part its proper warped, so probably the best thing is was to replace it…..which is my job for the weekend.  Any ideas what length the bolts are im assuming they are m10 hex bolts but not sure of length now they sheared…

M6x22, plus M6 spring washers