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Sounds more like a little valve ticking but nothing to worry about. I would do some more break in miles first ( 500 km ) before i would put my hands on the engine.

Engine noise sounds normal. Some primary chain and clutch chatter is all.
Did you vacuum sync the carbs? If not, it will improve the idle stability.

Thanks guys, good to hear This is the first bike (or vehicle for that matter) that I have worked on with carbs. It's also by far the oldest thing I've worked on. So I'm a bit unsure on what's 'normal'.

I have vacuum synced the carbs, but not since I put the new coils on - didn't think it would make any difference. Thinking about it now, there may be slight variance with all the cylinders firing properly (due to more consistent forces on the crank). Will check it again when I replace the fuel line next week (someone cut the line a bit too short... ).

Had another ride around the block today - trying to get a fix until I can get it out on the road
A couple of times it felt like the throttle wasn't returning properly. When I got back to the garage I checked and the throttle stuck open when I revved to about 4000rpm. I shut the bike off and tried again and they stuck with the engine off as well. I could see the slack in the throttle cable, so it's the butterflies that are sticking (Keihin CV34 carbs). The return spring is fine - it hurts my finger to hold the throttle open from underneath the carbs. It only seems to happen when the bike is well warmed up - even though the carbs don't warm up that much. It doesn't seem to be at a particular position as it has happened at 2000 and 4000 rpm in neutral. It also isn't consistent, so I don't think it has to do with thermal expansion (although this is the only explanation I have at the moment).
Not really sure how to diagnose this one.

It may require unganging the carbs and doing a thorough cleaning and lube on all the linkage parts. Hope it doesn't have to go that far to solve it.

I had another look and the butterflies are binding very slightly, enough to cause the problem. Guess I'll be pulling the carbs again... All things considered, it's not too bad. Considering I've never overhauled carbs before I think I can deal with this as it's been the only real problem so far

Pulled the carbs today and checked the throttle shafts. The coating was coming off the number 2 shaft and it was binding quite badly. The return spring would normally close it easily but I think when it warmed up a bit (wouldn't take much) it was causing the throttle to stick.
Replaced the throttle shaft and good to go now.

I did notice that the tip of the drain nipple on two of the float bowls was always wet. They weren't dripping, but if I dried them off then there would be fuel on them again after a while. I checked for leaks but couldn't find any (filled the float bowl with fuel, blocked the overflow tube off with my finger and blew as hard as I could through the drain tube). Is a bit of seepage normal?

There should be no weapage out the nipple.
Do your float bowls have real overflow tubes or not? If they have the tubes it's quite likely that some are cracked which can cause leaks.
If there is no overflow tubes the wet nipple means the drain screw is leaking.

They have overflow tubes. I thought they must be cracked, but my pretty primitive testing method didn't seem to work. Is there a better way to test for leaks?

Went back and tested one again. Must have had a bit more air in the lungs as I could see bubbles coming out of the overflow tube. They pretty much came out of the whole tube - the whole surface seemed to be porous. Looks like I need to replace them (soldering isn't going to be an option, I'd need to completely cover them.
I'll try to find some brass tubing and see if I can use your repair method.

I found 1/8" brass tubing at my local hardware store, sold in one foot pieces. His method of replacement worked like a charm.

Thanks, should I go slightly bigger or slightly smaller after converting to metric? Assuming slightly larger may mean I don't have to flare them.