Make sure you take it out of its plastic wrapper as soon as you can, and before you cook it, rest it on a plate lined with kitchen paper, covered with just greaseproof paper.

Let it sit at room temp for at least an hour before you cook it.

Dry the skin TOTALLY ie get a dry clean tea towel and thoroughly rub all over the joint to get it stone dry.

Then score the skin (to be honest, the best knife I have found for doing this easily is a craft knife!) and make straight clean score lines all over the skin around 1cm apart and 1 cm deep

Make sure your oven is hot. (200'c)

Add the joint to a roasting pan with just a little veg oil in the botton to stop it sticking

Just before you pop it in the over, rub salt all over the scored skin.

Cook in the hot oven for 30 mins per 500g meat.

That should do the trick! (Have you got gas or electric BTW? Electric is a 'drier' heat, so crackle tends to be even better in an electric oven...........)