Has anyone used an Optimus SVEA 123 stove before? Can you run them on normal unleaded petrol or do they have to be run on white gas?

Don't suppose anyone has one that they want to sell?

I had an Optimus 8R which used the same burner and it would run on any petrol no matter how crappy, although the worse the quality the more it smoked, it did run a lot cleaner on white gasoline if you have it.

This is serious old school technology, literally "when the going gets tough, the tough get going".
They are notoriously temperamental, and if you have incomplete combustion they produce a lot of noxious carbon monoxide, as well as other poisonous gasses. Get a good supply of "prickers", which are 0.3 mm wire in a handle, to clear the nozzle if it gets blocked/restricted by deposits from sub-optimal fuel or too low operating temperatures- lack of preheating for example.
That said, these stoves will be functional at high altitude and low temperatures, where most other heat sources can't function. In Norway we have a dedicated club to promote the use of these, and similar apparatus, the Primus Unionen, were members pledge to use these, you will be banned if you are using a gas canister. The annual Primus Treffen, last weekend in February, at Velmunden, near Bjørnroa, usually 200 + primus enthusiast attend. You have to be self-sufficient, there are no facilities at all.
The Svea 123 is iconic, if you know how to use the stove it will provide over 2000 watt of heat - enough to melt a telephone on a hotel bedside table- don't ask me how I know!
They are available on net sites, and there is a Chinese copy that has got good reviews, but originals are most often found in secondhand shops, auctions, flea markets etc.
If you can find one, and operate it safely, you become a member of an exclusive brotherhood. Good luck.

Peter, in Oslo