Tight valve locknut...

I was trying to do the valve clearances today. I'm no mechanic and this is the first time I've attempted such a thing. The engine has a reasonably loud tapping in the top end (consistent with the revs) and I hope the cause is, as I found out today, that the inlet vlave has a clearance of 0.15mm (at least) as opposed to the 0.10mm that it should have. However when I went to turn the locknut (anti-clockwise, as in toward the front of the bike) on the tappet, the nut simply would not budge. Is it normal for these to be very tight? - should I persist and place greater pressure? I could have applied more, but obviously this is one bolt in particular which I don't want to round, and one complex of metal I don't want to hurt, and as a first-timer I was anxious of any such possibilities! Some previous owner seemed to have a penchant for absolutely over-tightening every sing nut and bolt (singles do shake things apart!) which might explain things. Any advice - can it take decent pressure, and is there another method? (-if it were external to the engine, I'd give it a bit of WD40.) Also, is it fine to take the rocker covers off when the bike is running? (-I want to verfiy my sense that the tapping is mainly coming from the inlet valve tappet)

-Matt