Help please - overheated tonight

Hope someone can help. I've got a 54 plate ZT KV6 with 54k on the clock. On my 20 mile commute home tonight, I was sat in traffic in 25 degrees for around 20 mins crawling and stopping. Looked down and the temp needle was vertical and the red light on. Pulled into a lay-by and popped the bonnet. Steam was coming out and there was coolant around the top of the expansion tank..



No leaks underneath and I sat and let it cool for 20 mins, checked the coolant level and it was fine, still at the max mark on the expansion tank. No mayo and nice, clean and pink. Got in and drove home normally with the needle in its normal, half-way position.



I'm confused. Never happened before in the 9k miles I've owned it. Any ideas? Just puzzled as to the coolant still on max but obviously overheated. Could it be a sticking thermostat?



Cheers for any help you can all give me.

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