Defender 110 - loss of 5th gear - urgent help

Hi All - was driving 2 hrs ago on the M25 and during a gear change from 4th to 5th I appear to have lost the 5th gear on my Lt85 gearbox / Santana

I had to drive 20 miles in 4th, however a racketing nose had developed from the gearbox when any gear is engaged. Even more so on take off 1st / 2nd at lights...no noise when clutch pedal is pressed and all is in neutral...

I am now waiting to hear back from my local specialist - Insideout4x4 in Capel, in the meanwhile would any of you have any suggestion as to what the issue may be...some bearing failure, damage to the drive shaft?

Sugestions very welcome!

G

rear snap ring holding seal collar can come off this lets 5th gear slip backwards ,its possible layshaft gear for 5th has come off or shaft broken but former would be my first choice