I'll try to explain this as best as I can but i'm crap at this sort of thing so just bear with me!

My chain was very loose so i've had to tighten it up, but I've got a feeling it might be too tight now.

There is very little give in the chain, about 5mm each way (if that) , but I cant seem to loosen it off any more. As I assume every bike has, there are the two allen key adjusters on the back of the bike, one on each side of the back wheel. On the right hand one, loosening the screw doesnt seem to do anything, other than take the casing off. (I'm only hoping people know wtf I'm talking about here cause its a tw@ to describe!)

I need to keep it at the same point on the measuring scale thingy on each side so to counter it ive had to tighten it more on the left hand side. The measerer thingy is at the same point on each side so i'm assuming everything is lined up ok, but the chain is very tight.

I've been out on it today and It handles much much better than before, feels much more responsive and gear changes are swifter and smoother, pulling off seems faster too , but the thing that concerned me is when i pull the clutch in and let it roll along it makes a strange whirring sound which wasnt there before.

Does anybody understand that enough to give me any advice?

Some of the Bandit owners will be along in a minute for sure

But most chains should have about 25mm slack that is mid way along the lower run and taken as an upwards measurement only. Have the bike on the centre stand or paddock or whatever and spin the rear wheel to check at different points along the chain, you should take your final measurment at the point that has the least movement (chains have tighter areas in them ).

you are correct to adjust so the adjusters line up equally but as for the fine detail as to how yours works without a pic I don't know

You definitly have it too tight, its worse to have it over tight than it is to have it loose, you could knacker your chain in maybe 100 miles. when you sit on the bike the suspension compresses, this tightens the chain because the swingarm pivot is slightly above the line from the front sprocket to the rear. so you'll probably find the chain is very taught when you sit on the bike, not to mention when you go over a bump so your gonna stretch the chain or even break it!

To loosen the chain up you have to loosen the adjuster bolts as you said and then give the back wheel a good kick from the rear to push it back into position (you may need to kick it several times) then just check the chain again and make sure the marks line up. also just do a quick visual check to make sure the wheel is aligned properly as sometimes the marks are unreliable!

make sure the wheel has not moved out of alignment after you tighten the axle nut as it can sometimes move while tightening. to stop this always make sure you only push vertically down towards the floor with the spanner and not horizontally if you know what i mean.

and while you are at it,,,

give it a good clean & lube it

and check for any tight spots......or any twisted sprocket.....

you have slackened the rear spindle 1st havent you? spin the wheel & find the chains tightest spot, then undo the nut/bolt on the rear spindle(axle) you want to aim for at least 25mm play on the BOTTOM run of the chain. make sure the adjusters are lined up equally, then tighten up the spindle again.

by "spindle" he means "axle" ie. the big bolt that runs through your back wheel - I must admit I'm a bit confused by your decription of not being able to loosen the chain - does sound like it's too tight mind....

Yeah thats all loosened off propperly. I'll try and get a photo of what I mean about the right hand bolt - I cant really describe it.