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If running good above 3,000 rpm, it may be best to leave the main jets as they are. Just opening the pilot screw another turn or more may take care of the problem. If not, a larger slow jet is what you need. Attached is a carburetor circuit chart.
I have found that when adjusting pilot screws there is not an exact spot where the bike idles best, but about a 1 turn range. Living at sea level also, I tend to keep my carbs adjusted to the lean side of that range. When going on rides, I can only go up from here.

Excellent advice as always guys, thanks a lot. Yeah bike is bog standard and that's the way I wanna keep it. In fact its frighteningly original, hence why I want to keep it that way. Everything previously mentioned all checks ok, so I think it just needs a bit of fettling..

Tell you what I have just noticed. The choke rod operates 4 brass plungers , one on each carb, that have black rubber seals ? If they're meant to be soft and rubbery, mine aren't. They're rock hard and broken. Could that be causing an air leak and leaning it out ?

The external rubber seals you can see are dust caps.
It's possible for air to leak in past them and enter the carb past the plunger and weaken the mix throughout the rpm range..
They are readily available from ZPower etc in the UK.
If you are referring to the internal rubber seat on the end of the plunger then these will cause rich running if worn.

Thanks zed 1015. Its the external ones that are buggered !!

Gave the pilot circuit a good clean out with carb cleaner and compressed air today. On reassembly I noticed that the plug that fits over the pilot jet is a pretty loose fit. My worry is, what would happen if they fall out down the line ? Are they important ? Should I carefully but firmly knock them in with a small hammer ?
Is this the right way round ??

Looks like new O-rings for those plugs is needed. They go in O-ring down, and the float bowl holds them in place and keeps them from falling out.

Thanks, can't seem to source the parts anywhere here. If I soak em in fuel overnight will that swell the o rings to make a better fit ?
Have heard you can put rubber plugs in but again can't find em !!!

The only thing I haven't cleaned out is the pilot screw and passage. Bit nervous about taking them out. Is it an easy job ? What exactly should be in there, is it screw, spring, washer , o ring , in that order ? If so I assume the o ring and washer will be stuck down in the bottom ?