Fill up a laundry tub (or use your bath tub) with warm (not hot) water and a little gentle detergent (shampoo is great for wool and silk - I learnt that from a Vietnamese tailor). Leave the jumper to soak for about 10-15 minutes.
Gently stretch and shape jumper to oit's original dimensions then dry it flat without heat (in the shade).
Give it a bit of a stretch and shape every hour or so while it dries (or not if you don't think it needs it).

It's the soaking that is important - it gives the fibres time to unlock/relax. So soaking not washing mmmkay.
THANK YOU!!!!! Will give this a go