Lie your pork on a chopping board and using a sharp knife, slice part way through about 1 cm from the base of the fillet.
Open it up (it is almost butterflied). Now starting from the centre, make another slice, again not quite all the way through, it needs to be joined together. Now open it up so it is like a book in 3 parts.
On your baking tray, line it with loose foil. Brush the foil with a little olive oil. Now lie the Jamon or bacon onto the foil, you need it to cover the same size area as your cut pork fillet.
Lie your pork fillet onto the jamon/bacon
Take 2 or 3 sausages and slice them open. Take out the meat and press this onto the pork fillet, leave a little gap all the way around the edges. Tuck the ends of the fillet in to help seal the ends.
You now need to roll the whole thing up like a swiss roll. You use the foil to tighten it as you do this. Once it is all rolled, fasten the foil tightly to keep the juices in and keep the foil seam to the top, again this helps keep the juices in.
Roast in the oven for about 40 minutes. Open the foil up for around the last 10 minutes to brown off the jamon/bacon.
Leave it to cool a little to allow it to set. To serve use a very sharp knife and slice into 1.5 cm chunks.
NB: You can make this in advance and leave in the fridge to really set, cut it and them warm it back up which is what I did on this occasion.