Hey guys, so I think I know what the answer will be here (head gasket/piston/valve problems) but I'm hoping I've missed something you won't

So, fitted a Milltek cat back and Janspeed header with decat pipe about 6 weeks ago and all was fine and dandy. Eventually I got an engine warning light come on after giving it some beans and after plugging in the diagnostics got a " P0420 - Catalyst System Efficiency Below Threshold" error. Not surprising, and no issue with running so left it alone.
I'd noticed for some time now that my coolant tank had evidence of leaking around the cap, so I bought a nice new genuine one and this seems to be the start of my problems.
The next day (and every day since) I've had a start up misfire. It seems to run on 3 cylinders, and if I leave it to idle I'll get the ASC light come on and the engine warning light and no power at all. I checked the code and it was a P0301 misfire on cylinder 1. I also get a big cloud of blue/white smoke and a strong smell of unburnt fuel.
If I rev it to 2k revs from start up for approx 5-10 seconds it will clear the misfire and run a treat - no error code, no ASC light, no loss of power and no cloud of smoke.
Once I stop the engine, say for 5 min or more, I will have the same issue, regardless of the engine being hot or cold.
I have a horrible feeling that as this seems to have happened since fitting the new cap that it is a head gasket problem

Any thoughts? Am I looking at a head off situation or could it be something less painful?

 

Is it consuming a lot of oil? If you're concerned with the possibility it may be a good idea to do a compression/leak down test.

 

update:

yes, it does use oil, about half a litre every 1000 miles ish - it leaks - got a nice oil patch on the driveway, which i think is from both the crankshaft sensor and the valve cover gasket.
I've checked the water level and this doesn't appear to have moved much, maybe down a little.
What is really frying my noodle is that for the past 2 days it has worked faultlessly - no misfire, no rumble, no smoke, nothing. This morning, absolutely fine, started cleanly instantly. Stopped at the shop on the way into work, started again, perfectly. Went to start it at lunch to go to the gym - clouds of smoke and misfiring.
What's going on??