judderbar said:

Struggling to see how that is white grease because you can see all the typical solid small parts of the same colour scattered on the battery

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Plus white grease wouldn't appear crystalised like this...

Cocothela said:

He said the white stuff is white grease not corrosion. And that their fault is saying it's in the steering ecu. And that it's not communicating properly with the engine ecu. But it drives fine and apparently would pass another mot in this condition so I guess it's okay to just assume it's old...

Car dementia, forgets it can talk to itself poor thing

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Steering ECU fault would illuminate the steering light not the EML, plus if the Steering ECU packs up you basically just lose the steering assistance.
Don't see how a steering issue is gonna make the engine run rough.
Personally i would go with what Ziggy said...clean the Earth lead up on the battery and where it connects to the body. Earthing issues on Punto seem to cause all manner of weird symptoms (example rear light cluster earth can cause probs with instrument panel).....

That doesn't look like grease to me!. Invest in one of these for the battery and leads well worth it.

It's been running fine up until now, sometimes unhappy to start, but tonight I've left work and it won't start at all. Lights come on, it tries to start but won't catch. Not happy seeing as I've been told twice now that it's fine. Waiting for the breakdown company to come out

Took him a couple of attempts but it started, he said it seems electrical and mentioned he wasn't sure if he heard the fuel pump on one of the false starts, and said to take it to a garage again.

To add insult to injury once I got home i tried it again and it started perfectly, no hesitation at all!