I used to have a corsa - similar age, but with a petrol engine.
My issue was that I had to push in the speedo panel, in order to join some connectors so the car would start. This had to be done every time in order to start the car. I was told (by a repetutable vauxhall repair garage) that for some reason there are issues with the wires coming loose in that area, and for me the problem was solved by having the panel removed, all wires disconnected, then re-connected.
I'm not saying this would necessarily fix your fault, but it is possible that this repair has been done/attempted in the past.
A google search for "corsa dashboard electrical fault" brings up many results.
Good luck, just though I'd put in my experience. :)
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