If the problem aruses when the cold start unit resets, I would be looking specifically at the cold start valve operation not working properly and maybe causing over fuelling..
When it starts to mis fire, is there an increased smell of petrol under the bonnet and black smoke from the exhaust as they are both clues to this issue.
Also backing up Jon M comment above…when E10 was introduced, the 4 cylinder Rover engines were on a list that I seem to recall was going around some of the dealers, and were thought to be incompatible. They didn't say in what way though, so treat it as a red herring in your case as there must be plenty out there that are still behaving fine.
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