Ok - so there isn't a swappable 'open differential' alternative available.
Ok - so that option isn't avavilable either!
My old transmission has done 100k and having read various articles about them the viscous couplings often only last about this long so I am slightly reluctant to swap it as I may have the same problem again in a very short time!
Fairly certain it is the centre viscous coupling:
Test 1
Turning the stering wheel, whilst manoeuvring at low speeds, feels like gently applying the brakes; i.e. the transmission is binding.
Test 2
Front weels off the ground (rear wheels on the ground), with the transmission in nutral, I can't rotate both front wheels in the same direction at the same time.
Thanks for your advice "whitetiger" - looks like I will just have to 'bite the bullet!'
Thinking about why it may have failed - I did get a puncture about 3 or 4 months ago where I had to put the 'space saver' spare wheel on and drive ~30 miles. As the space saver is a smaller diameter this will have caused the outputs of the centre diff to be spinning at different rates and heat up the fluid which ultimately could have cause the failure I now have.
If this is the case why do Subaru put space saving wheels in their cars? ...because if they are ever used they are likely to result in centre viscous coupling failure! Do they warn about this in the owners manual (unfortunately I don't have an owners manual to check).