Car suddenly having trouble engaging gears

[Polo Driver]

While I was out driving, moving away from traffic lights, the car suddenly wouldn't engage 2nd. I got up to speed in third easily, but then 4th wouldn't engage, 5th did, again easily.

The clutch pedal is going in and out easily and I was able to stop at lights and move away again in 1st. When I got back to the house I tried reverse but it felt weird and it seemed to make any other gear difficult to engage.

Any idea what this might be caused by and what sort of bill I could be looking at? It's a bog standard 1.2 diesel with a 5 speed manual.

Update of sorts, it feels like the gate under the cover of the gearbox has dropped, neutral is where 2-4 lie and 1-3-5 are all where the neutral position was before. Almost like something has slipped down towards the handbrake.