Very nice bike.
So, to be terse, on to the questions.
The main jet is, indeed missing. Are you certain it wasn't in the bottom of the float bowl when you took the bowl off? Nope, wasn't in the float bowl ...........but - confession time now - I found it on my bench!
If you were not the one to replace the phillips screws with allens, then whoever was there before you seems to at least have some inkling of doing things correctly, so it's quite odd that the main would be AWOL. Replacement allens were me.
Of course, the main should be replaced. If the UK is anything like the USA, the jets should be available at almost any dealer. Make certain to get the correct size. It should match the other main in number.
On the tiny brass tube, the SR41s (750s) only had a "choke" (enrichener circuit) in the left carb. Unless I'm mistaken this tube provides the fuel pickup for that circuit. The same spot in the right carb should be blanked out and not drilled for anything, correct? Correct - I should have spotted that.
Now, for the fuel leak. Unfortunately, the answer to your leakage question is "None of the above" Thanks very much Mr Suzuki for not mentioning any of these O-rings in the troubleshooting section of the workshop manual under "fuel leakage".
Fuel is leaking because one or more of the small O-rings that these carbs use is dried out/cracked/torn/square in cross section/or otherwise damaged.
You'll find these O-rings
(a) under the brass tube that holds the float valve Replaced
(b) on the plastic "tree" that supports the floats themselves Replaced
(c) on the bottom of the grey plastic throttle slide guides where the guides go into the carb body.
If so, how should it come out?
And the other place may be the float valves themselves. If the small black taper is dented or damaged, fuel will overflow since the floats will not stop it. From here it usually fills the crankcase...which it much worse than leaking out of the carbs. Replaced
There are also O-rings on the tiny idle mixture screws down at the bottom...they are very very small but should be replaced. Replaced today - haven't re-tested for leakage yet.
Replacing all these bits is not expensive but is a bit tedious. The bits for the grey plastic slide guides I was able to find from BMW or KTM since a few of their bikes use the same parts. The Suzuki parts diagram does not even list the slides or the O-rings as a seperate part.
Setting the float level correctly and replacing all the O-ring should solve your fuel leakage problems.
BUT...you may also want to check the fuel "T" that lives between the float bowls. There is a seal on each side of this "T". If the "T" spins around in the space between the bowls with little to no effort, these seals should be replaced. The "T" should be able to rotate but should have significant "drag" when moving it. If it seems loose, fuel will leak from that part also. Tested on the bench connected to a gravity fed fuel source - no leak and sufficient "drag".
Once all these problems are solved (a bit of what's required in owning a 20 year old machine) the carbs work quite well.
Lots of information and photos if you do a search for "carbs" on this thread.
Remember to use the fuel valve on the tank and turn the fuel OFF after you put the bike away for the day. Leaving this valve on is asking for problems.