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.
West Yorkshire Area Organiser & forum moderator
Meetings take place on the 1st Wednesday of every month at The Hollies Sports & Social Club, 12 Hough Lane, Bramley, Leeds, LS13 3NE
Dolomite 1850 auto (NYE 751L - The rolling restoration)
Citroën C4 Grand Picasso 2.0 HDi Exclusive (MA08 WCL - the modern)
BMW 318i (M265 PNC - Project Bimmer)
Vauxhall Corsa Design 16V (FE04 FKG - not mine! Being revived)
Former stable of SAY 414M (1974 Toledo), GRH 244D (1966 1300fwd), CDB 324L (1973 1500fwd), GGN 573J (1971 1500fwd), DCP 625S (1977 Dolomite 1300) & LCG 367N (1975 Dolomite Sprint) plus 5 Acclaims and that's just the Triumphs!
Check my blog at http://triumphtoledo.blogspot.com
"There is only one way to avoid criticsm: do nothing, say nothing and be nothing."