Sam, this reminds me of when I was doing the engine swap on a Corsa. I had to change the head gasket as well and as I was fitting the timing chain (the 1.2 engines have a timing chain) the tensioner which was under compression came off and ended up in the sump. Fortunately, I managed to fish it out with a telescopic magnet. The bottom of the timing chain cover mates up with the sump and the 1850 engine is similar. I've been looking in the parts catalogue and the shallow part of the sump is under the timing chain so the nut might be resting on there if you're lucky. Keep on trying with the magnets and if you still have no joy then it will be sump off to retrieve the nut.
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