Welcome to the forum PhilBert, nice to have another HGT owner with us.
You could of course just buy a entire new rear swing arm but if your wheel came of the way I think it did your also probably also lost the spacer (part 3 in this diagram). You will need the correct thickness spacer to give your wheel the right offset.
You can check for part numbers and basic exploded diagrams in
eper. Put in your VIN number to get the right part numbers for your car.
http://eper.fiatforum.com/
If you just want the stub axel/the threaded pin. Fiat don't sell it separately to the swing arm which is daft. I think you will get the swing arm new from fiat, lots on ebay or any motorfactor.
Shop4parts sell new arms here;
http://www.shop4parts.co.uk/?name=s...o_2.0_20v_Suspension_Arms&cat=Suspension_Arms
This picture nicely shows the swing arm with the stub axel going through. You can make out a large hole for a hex head in the axel. I have seen it with my own eyes when a independent mechanic does not realise the Bravo axel needs to be locked with a large hex socket to stop it spinning when tightening and loosening the hub nut. Fiats service manual clearly shows a hex socket locking the axel.
potentially lots of Fiats could use the same stub axel but since fiat don't list them as an individual parts there is no way to compare part numbers, you would have to check them physically. The cars that will most likely have what you need would be, the Tipo, Marea Multipla, Coupe, Brava and Bravo. Marea 20v would be best.
Maybe give a motorfactor a call, I was sure the stub axel used to be on eBay new regularly.
I've just bought a 155 20v hgt but need a new stub axle . Does this have to be a specific part or can I get one of a different car like a punto for example . Can anybody please help me. I absolutely love this car and was gutted when the wheel came off and flew the road