Series 3 Gearshift Jammed in Neutral on S3

Before I start pulling things to bits I thought I would glean some insight from others who are likely to have better ideas/experience. I have S3 diesel 1982 (all standard), pretty reliable, no previous gearbox or clutch issues, strange noises etc. Reversed out of shed this morning, no problem engaging reverse gear, then went into first no problem to go a few feet forward to get a better angle past a parked car, but really struggled to get out of first back into neutral and then couldn't get back into reverse at all. Gear lever will not move fore and aft at all with only a little sideways movement from neutral position. It's really solid and no hint of movement even with a fair bit of force and with clutch depressed- same with engine running or not. No clunks or strange noises during entire sequence. Gearlever will move up and down on the spring as it does when you go to engage reverse but won't go completely to the left or fore and aft. Plenty of clutch fluid and no signs of a leak from around the slave cylinder although the clutch pedal does seem slightly lighter than normal. Any suggestions gratefully received - I'm guessing either slave cylinder (although would expect at least some ability to go into gear) or, more likely, selector mechanism issue in top of gearbox. I have tried a search on the forum bit struggling to find anything relevant so any pointers would be great before I start unscrewing things to have a look. Many thanks........Ian (currently regretting fitting copious amounts of Killmat sound deadening and rubber matting to entire cab floor over gearbox access panels!!)

Clutch pedal lighter suggests an issue with it. Possible fail of master cylinder seal [ black in reservoir a sign] However one should still be able to select a gear or two with engine off.
Have known a stone to fall in just the right position to jam selectors and end of stick to break.

If stick won’t move fully sideways in neutral I wonder if two gears engaged same time, taking the top off might show what’s going on

I've had a broken synchro spring give me grief like this, but lots of wiggling of the lever let it fall free. I didn't know what caused it at the time, but all became clear on the next oil change.

Judging from the killermat, check if you have plenty of clearance under the pedal. Maybe you fitted some and now you cannot fully engage it properly? Still wouldnt explain the weird "lockup".

with aftermarket selectors, the O-ring at the end of the selector is different and can therefore sometimes slip between the gear forks. this will give you a really loose gear shifter and you wont be able to engage anything. But this also doesnt seem to be the case.

As mentioned before, probably just check ur slave and master. Possibly only replacing the slave? but if the slave failed, the master cant be far behind.

Thanks for the replies so far - haven't had time to get at it just yet but some good ideas of what to check for. Just before I dive in, can someone just confirm that on a normal S3 you should be able to get into at least a couple of the gears from neutral even if the clutch is engaged (ie pedal up with engine off). On all of my other old stuff its possible, although a bit baulky. Just trying to rule clutch out of the equation before I start taking floor up. Could start with clutch slave/master cylinder I guess but sneaky feeling its more than that.

Yes one can usually find a gear or two with clutch out, engine off and at a stand still but given your stick will only move a small amount I guess other issues as noted in posts above.
First thing out with floors and tunnel to view selectors on the top of the box.
Britannica restorations and or Steamwally for videos of what you will be looking at.

Thanks for quick reply tottot - I'll let you know what I find.

Good luck.