Mountfield mower problems
 

Hi, I've got a Mountfield Emperor 83304 with a Briggs & Stratton 111787, 0162-01, 1985 model. It is stuck in 'driving' mode with the drive roller continually turning, ie. the clutch does not disengage, and it just drags me round the garden!!

I'm reasonably handy with tools so would like to have a go myself but have no manuals... The operating handle/cable is fine and the clutch lever which goes into the engine casing is moving freely - it would seem that something has gone awry inside... Any hints on how to fix it?? Can't see how to get to the innards without completely splitting the engine and didn't want to go to those lengths without some sort of second opinion...

Any help or advice appreciated