If the rectifier is shorted, it will have symptoms like you describe. Disconnect the leads from the transformer to the rectifier assembly and see if it still draws a heavy load.
Have stripped the contactor down and it was very dirty so have cleaned this with servisol. Have managed to temporarily bodge the rectifier together for a test and it's working again so thanks for your help. Have just ordered a new rectifier. It's cost me a few bob but I've learnt a lot about how welders work so it's all good.
