The thermostat cap is leaking.

It's strange it didn't leak while the engine was on and flushing but I turned the engine off and squeezed the top radiator pipe (connecting to thermostat housing) and there was no pressure and air and water escaped through the cap and housing of the thermostat, I have no thermostat in there atm but that's only becuase it's being flushed.

This is how I flushed it:

1. Drained old coolant.
2. Removed thermostat housing (cover and took it off cylinder head).
3. Took out thermostat
4. Put a new gasket on (between thermostat housing and cylinder head)
5. Put the thermostat housing back together with no thermostat with no thermostat or rubber ring seal that sits ontop of the thermostat.
6. Attached everything back up (tightened thermostat housing to 20nm like gregory's says.
7. Added flushing liquid and water.
8. Put the Surge tank cap on only 1/2 way so it wasn't closed.
9. Turned engine on and flushed.
10. Turned engine off and found problem.

Also I can't seem to get the drain plug out again it won't hurt if I drain the flushing liquid tomorrow morning will it?

There is no issue here matey. They don't normally run a gasket. The o-ring around the thermostat is the only seal. Thats why it leaked. When you put the thermostat back in with the seal it won't leak.
As far as leaving the flushing liquid in there: all I can say is go by the directions.