Technically I don't know what sort of vinyl is used. Dash tops pre about 1964 were thicker and have a coarser grain pattern. These survive much better than the later ones which are thinner and have less patterning. I think it was just the way they were moulded. The late type had the grooves pressed into the mould which makes the material there much thinner, hence the cracks. The foam on the backs is different too. If yours is good now I'd just keep the sun off it, perhaps with a cover if parked up in the sun. RHD covers can be bought in Australia for many cars. Don't know if they do Amazon ones but certainly for 240 and these look quite good.
I don't know if any of the liquids offered to protect vinyl will actually do much of a job over a long period. When Armorall first came out it was supposed to be the wonder spray. The first type made the dash very shiny and it had a lot of silicone in it. An alternate less glossy version came out later but neither got very good press over time for this job.
A heavy duty spray formulated for marine use might be the way to go. Maybe try
http://www.ebay.co.uk/itm/303-Aerosp...kAAOSwRLZT2jib
http://www.amazon.co.uk/3M-Cleaner-C...r%2C+Protector
Having said that Autoglym vinyl and rubber care probably works fine and is MUCH cheaper.
Bottom line, keep the sun off it.