Goes into gear but wont move.

So a few months ago, on my way home from work, my bike broke down. 
I was accelerating up the road, when i changed into 4th, but the bike didn't. Seemed like it just gave up on the idea of changing gears. It gave a whirring (almost grinding) noise whdn the clutch was engaged, but as soon as i pull in the clutch or put it back into neutral, it sounds normal (even while rolling).
I have changed the oil, no metal on the plug. I have changed the clutch plates. I have adjusted the clutch cable.
I am not a mechanic, not mechanically minded, so please feel free to dumb down responses :) . 
If i put it in gear now, there is a whirring (almost like when you cant quite find the gear in a car, but without the grinding) from the front end. If i roll it forwards while in gear, the noise doesnt change and it wont pop into gear. If i roll it backwards, there is a bit of a grindy noise comes from the rear wheel where the shaft is.
This is driving me insane and the local suzuki garage says they would need to strip the engine (to the tune of at least £1000). 
Has anyone had this issue or know what i should try next?

Sounds exactly like what happened on mine..... splines on shaft drive / diff had worn. 

My bike repair place managed to get a used replacement and it sorted it. Not cheap tho!

I can't guarantee that's the issue with yours, but it might point you in the right direction.

Good luck!

By the way Billy, if you can be bothered wading through pages and pages, here's my write up on the problem, pluspicsof the damaged parts...

https://www.suzuki800.com/t5971-aaarrrrgggghhhh-breakdown

That sounds like the exact same issue im having. Is it a job that a "you tube mechanic" can do? (as long as i have you tube and an internet connection i can fix anything ) or would you recommend taking it to a bike shop?

If it's the shaft, it's fairly easy.  However, you'll need to be able to get the back wheel out, remove the final drive unit, then remove the shaft itself.  Quite a few members have done it when they did the 1400 conversion - sure they'll point you in the right direction

AS Dave says it's not that hard a job really.
brief description ok:-
Get bike at least 3 inches off the floor at the back, this may mean you & 2 mates doing the job ok?
Remove the back brake rod & torque bar from the back plate, no need to remove the brake actuater arm.
Remove the rear wheel spindle & spacer. You should be able to remove the wheel now or at least move it to one side so it is off the splines.
Remove the drain plug on the diff unit & drain the drive as you'll need to replace the oil anyway.
remove the false swing arm cover if it's still on, only a few bolts, & then undo & remove the 3 nuts holding on the diff.
You may have to use a bit of wood to 'tap' the diff with if it's stuck to the drive tube & remove the diff.
Clean up the housing(diff) & then you'll be ready to look at the drive shaft after removing the oil seal cover(you will possibly destroy the old one anyway) & the internal circlip that holds it to the diff if I remember rightly & the drive shaft will now come off. You will have to order a new seal & circlip to ensure you get the best fit etc.
At this stage you will see if there is any damage to the drive shaft & if you'll have to replace it or whatever?
Rebuild is a bit like the break down but in reverse & do not forget the moly grease on the splines of both the drive shaft/diff & the wheel splines in the back of the diff.
I'm sure you will be able to download the workshop manual from the forum library & if you get stuck we're here to help you out ok.
I did the VS1400 diff swap but it was a few years ago now but it will be a useful place to search for more info to help you out & it's on the forum, type in VS1400 diff swap in the search section & it will come up ok.

Does the year of the bike make a difference?
I can find plenty of second hand drive shafts for an 05, but none for an 09.
Also, is there any difference between the VZ and the VL in terms of the final drive?
You guys are a great help by the way. Thank you so much

Check the part number (try CSML) - I believe they all use the same shaft. I'll have a look later

Just had a look at MSP - it's the same part number for the 05 VZ, the 09 VZ & the VL.  May be worth having a search on ebay using the part code or looking through MSP & seeing which other bikes use the same part.

Same parts from 05 to today for a VZ/M800, A VL/C800 years 01 to today will fit as wel.

As for there being none broken in this country, well they are but few & far apart.
You can get the new replacement parts from a dealer, I use Crooks Suzuki(google it) as they know what you'll require & get excellent service as well.
I only waited a few days for some bits as they came from Germany for the 1400 diff but std parts are normally next day or so ok?
As Dave said try CMSNL.COM for an online parts list & the will also tell you if they have the part in stock as well.

As promised. My update.
it was the shaft. I bought a used one off ebay (05 model) , arrived this morning. Got a few mates round to help. Took the back wheel off, got the final drive off, bunch of rusty flakes came out the shaft cover, shaft was loose and wobbly. Didn't look good. Put the new one on, put the bike back together, fired it up, put it in gear, drove away. 3 hours work (would have been a lot quicker if we actually had the right tools and had any idea what we were doing haha) 6 cups of coffee and a second hand shaft. The Suzuki dealership had it for a week, £300 worth of labour charges, apparently they checked and ok'd the shaft. 
I believe they will be hearing from me very soon.

If anyone else has this issue, please check this before doung any engine stripping. The garage wanted me to spend over a grand to strip down the engine and check the gear box.

Fucking cowboys - wanted me to out over a hundred quid to buy a "specialist" tool for them to do the job for me. Borrowed a wheel from Pete - worked just as well. Well done on a job done well

Glad you got it sorted     Don't start me on 'Dealers' 

So, emailed the dealership explaining (fairly reasonably i believe) why i am angry and what i want done about it. Also added the general manager of Suzuki UK to the email. Just to add a touch of severity to the email. I wonder who i will hear back from first. Any guesses?

Hope you put some moly grease on the splines or you'll be doing it again soon!!!