1980 Austin Princess mild custom - 1951 Lanchester LD10

It's amusing seeing little bits in the Honda interior of a totally different car that are just like what's in the Rover. The petrol flap and boot release unit on the inner sill there looks exactly the same as the one on the Rover. If you hadn't said, I wouldn't have known that carpet wasn't factory in there, it looks right against those seats and nicer than the usual grey or black.

R8 seats best seats. The only seats more comfortable for me driving long distance have been BX seats, but I wonder if some of that was helped by the sublime BX suspension. Princess seats are nice short distance, but they haven't a lot of support so you tend to end up a bit hunched after a hundred miles or so.

Have you checked the new sender has the same pressure rating as the old one? Some manufacturers apparently colour code them.

Good news and odd news today. Good news is that the new clutch hose was in fact correct this time and by the time I'd done my errands and got to the unit, Mike had already fitted it. The other good news is that the new hose made bleeding the clutch hydraulics much easier, lending credence to the theory that the old hose was knackered.
20180321-01 by Angyl Roper, on Flickr

The odd news is the clutch. I'm going to break this down into a little list to hopefully explain it a bit better, there might be something obvious I'm missing.

Gear selection with engine running - No gears will select. Fourth is the only one that makes an audible protest with a grinding noise, 1-3 and reverse won't even let you put the stick in the gate.
Gear selection with engine off - perfectly fine.
Starting car in gear - perfectly fine, in all gears. This was done just with the gear selected and a flick of the key, I didn't try to drive it like this.
Pedal feel - positive and like a clutch should feel. Pumping the pedal makes it very firm and difficult to push, as you'd expect of a healthy system.
From the information in the various manuals we've got with the car, Mike and I suspect either something in the clutch is stuck or the clutch itself is worn out. I don't especially want to pull it apart and do a clutch change at the moment, especially if it's a big or expensive job.
Clutch arm operation - as described in the book with no adjustment left to make and no obvious signs of excessive wear that might reduce travel. Even operating the arm manually (big pry bar) to force maximum mechanical travel and thus remove the hydraulics from the equation resulted in the same lack of gear selection when engine was running. This is why I don't think it's hydraulics related or air trapped in the system.

I'm actually annoyed about my mixed feelings about this car at the moment too. I did find if I sit on only half the driver's seat with my bum nearly on the handbrake, I can actually use all the controls comfortably. That's not ideal for actually driving the thing. It started up on the first flick of the key, even after being left alone outside for ages in all weathers. It's such a willing and charming little thing I'm disproportionately sentimentally attached to it! I am really fond of this little car and while realistically it is going to have to go before I move house this year, I really don't want it to go to someone that's just going to pull it to bits or worse. This is stupid because it is just a car, obviously, but I have the feels and like the Princess, there's something about this little car that makes me want to persist with it.

Big job to sort? I don't really want to be getting into clutch changes and the like on this given the time I've got available.

While I agree with everything you said, I'm not convinced that the clutch isn't stuck. I've started cars in gear multiple times when the clutch hydraulics gave up a long way from home.

It all sounds like a stuck clutch to me.

I think I might have confused things. If you start it in gear, it moves in the direction of that gear, in all five gears. You can't depress the clutch pedal or the clutch arm to stop it doing this.

If a strip down is the only option, it’s not a massively difficult job. It’s at least thirty years since I did a 1300 clutch, but if I could manage it at eighteen, in the road outside my mate’s parents house, in the pouring rain, I’m sure you’ll be fine.

This all sounds very promising, thanks guys I won't get chance to faff about with it until Friday at the earliest because of work commitments but I'll definitely give this a go.

We get this on the 40's lorries a lot. I had to do 4 laps of the field in the Austin K5 gun portee before it would let go, and that's a heavy thing with a 17 pounder gun on board. If possible have another go, it's certainly easier than taking it apart